feat(imagegen): optional per-request generation settings

Add Steps, CFGScale, NegativePrompt, Sampler, Seed to imagegen.Request
(pointer/empty = leave the backend's per-model default), with mirror
options, and forward them in the llamaswap wire payload as the
stable-diffusion.cpp fields (steps/cfg_scale/negative_prompt/
sample_method/seed). Unset fields are omitted so sd-server keeps its
baked defaults.

Lets callers (e.g. mort drawbots) override only what they explicitly set.

media/media.go

@@ -5,10 +5,16 @@
 // already satisfies the target's llm.Capabilities. Images that do not fit
 // are decoded, downscaled (never upscaled), and re-encoded into an allowed
 // format and byte budget. Anything that cannot honestly be made to fit —
-// undecodable formats, impossible byte budgets, too many images, images for
-// a text-only target — fails with an error wrapping llm.ErrUnsupported so a
-// failover chain can advance to a more capable target without a health
-// penalty.
+// undecodable formats, impossible byte budgets, images for a text-only
+// target — fails with an error wrapping llm.ErrUnsupported so a failover
+// chain can advance to a more capable target without a health penalty.
+//
+// Over-count is the exception: a request carrying more images than
+// MaxImagesPerReq does NOT fail — the oldest images are replaced with a short
+// text placeholder and the most-recent MaxImagesPerReq are kept, because a hard
+// refuse exhausts a chain whose targets share the same cap (e.g. an agent loop
+// accumulating a preview image per iteration). MaxImagesPerReq remains the
+// per-model knob (0 = no image support).
 //
 // Why a separate package: every provider would otherwise duplicate the same
 // decode/scale/encode pipeline. Providers keep only a cheap capability
@@ -52,15 +58,21 @@ func Normalize(req llm.Request, caps llm.Capabilities) (llm.Request, error) {
 	if !caps.SupportsImages() {
 		return llm.Request{}, fmt.Errorf("media: %w: target does not accept image input (request carries %d image(s))", llm.ErrUnsupported, total)
 	}
-	// Why error instead of dropping the overflow: silently removing an image
-	// changes the question the caller asked; the honest move is to refuse and
-	// let a chain try a roomier target.
+	// Over-cap images are elided in the same copy-on-write pass below: the
+	// OLDEST excess are replaced with a placeholder and the most-recent
+	// MaxImagesPerReq kept (see the package doc for why we elide rather than
+	// refuse). toElide is how many of the first images, front-to-back, to drop.
+	toElide := 0
 	if total > caps.MaxImagesPerReq {
-		return llm.Request{}, fmt.Errorf("media: %w: request carries %d images, target allows at most %d per request", llm.ErrUnsupported, total, caps.MaxImagesPerReq)
+		toElide = total - caps.MaxImagesPerReq
 	}
 
+	// Single copy-on-write pass: for each image, the first toElide become a text
+	// placeholder; the rest are size-normalized against caps. The Messages slice
+	// and an affected message's Parts slice are copied at most once.
 	out := req
 	copiedMessages := false
+	seen := 0
 	for mi := range req.Messages {
 		copiedParts := false
 		for pi, part := range req.Messages[mi].Parts {
@@ -68,6 +80,12 @@ func Normalize(req llm.Request, caps llm.Capabilities) (llm.Request, error) {
 			if !ok {
 				continue
 			}
+			seen++
+
+			var replacement llm.Part
+			if seen <= toElide {
+				replacement = llm.Text(imageOverflowPlaceholder)
+			} else {
 				norm, changed, err := normalizeImage(ip, caps)
 				if err != nil {
 					return llm.Request{}, fmt.Errorf("media: message %d, part %d: %w", mi, pi, err)
@@ -75,6 +93,9 @@ func Normalize(req llm.Request, caps llm.Capabilities) (llm.Request, error) {
 				if !changed {
 					continue
 				}
+				replacement = norm
+			}
+
 			if !copiedMessages {
 				out.Messages = make([]llm.Message, len(req.Messages))
 				copy(out.Messages, req.Messages)
@@ -86,12 +107,17 @@ func Normalize(req llm.Request, caps llm.Capabilities) (llm.Request, error) {
 				out.Messages[mi].Parts = parts
 				copiedParts = true
 			}
-			out.Messages[mi].Parts[pi] = norm
+			out.Messages[mi].Parts[pi] = replacement
 		}
 	}
 	return out, nil
 }
 
+// imageOverflowPlaceholder replaces an image elided to fit a target's
+// per-request image cap. It keeps the message turn intact and tells the model
+// an earlier image was omitted rather than silently changing the conversation.
+const imageOverflowPlaceholder = "[earlier image omitted to fit this model's per-request image limit]"
+
 // Info reports an image part's sniffed format ("jpeg", "png", "gif", or
 // "webp") and pixel dimensions. It is a cheap metadata read — the pixels are
 // never decoded. webp is recognized by signature but not decodable with the

media/media_test.go

@@ -149,18 +149,48 @@ func TestNormalizeImagesUnsupported(t *testing.T) {
 	}
 }
 
-func TestNormalizeTooManyImages(t *testing.T) {
-	img := llm.Image("image/png", encPNG(t, gradient(4, 4)))
+func TestNormalizeOverCount(t *testing.T) {
+	// 3 distinguishable images across 2 messages; cap = 2. Over-count no longer
+	// errors — the OLDEST image is replaced with a placeholder and the most-recent
+	// two (the relevant ones in an iterative run) are kept, in order.
+	a := llm.Image("image/png", encPNG(t, gradient(2, 2))).(llm.ImagePart)
+	b := llm.Image("image/png", encPNG(t, gradient(4, 4))).(llm.ImagePart)
+	c := llm.Image("image/png", encPNG(t, gradient(8, 8))).(llm.ImagePart)
 	req := llm.Request{Messages: []llm.Message{
-		llm.UserParts(img, img),
-		llm.UserParts(img),
+		llm.UserParts(a, b),
+		llm.UserParts(c),
 	}}
-	_, err := Normalize(req, llm.Capabilities{MaxImagesPerReq: 2})
-	if !errors.Is(err, llm.ErrUnsupported) {
-		t.Fatalf("err = %v, want ErrUnsupported", err)
+	caps := llm.Capabilities{MaxImagesPerReq: 2, MaxImageDimension: 64, MaxImageBytes: 1 << 20, AllowedImageMIME: []string{"image/png"}}
+	out, err := Normalize(req, caps)
+	if err != nil {
+		t.Fatalf("over-count should not error: %v", err)
 	}
-	if !strings.Contains(err.Error(), "3 images") || !strings.Contains(err.Error(), "at most 2") {
-		t.Errorf("err message %q lacks the counts", err)
+	var imgs []llm.ImagePart
+	placeholders := 0
+	for _, m := range out.Messages {
+		for _, p := range m.Parts {
+			switch v := p.(type) {
+			case llm.ImagePart:
+				imgs = append(imgs, v)
+			case llm.TextPart:
+				if v.Text == imageOverflowPlaceholder {
+					placeholders++
+				}
+			}
+		}
+	}
+	// The exact survivors are the most-recent two, in order: b then c (a elided).
+	if len(imgs) != 2 || !bytes.Equal(imgs[0].Data, b.Data) || !bytes.Equal(imgs[1].Data, c.Data) {
+		t.Fatalf("kept %d images; want exactly [b, c] (the most-recent two)", len(imgs))
+	}
+	if placeholders != 1 {
+		t.Errorf("placeholders = %d, want 1 for the elided oldest image", placeholders)
+	}
+	// Input request untouched (copy-on-write): the first part is still image a,
+	// not a placeholder — a len check alone wouldn't catch in-place substitution.
+	first, ok := req.Messages[0].Parts[0].(llm.ImagePart)
+	if !ok || !bytes.Equal(first.Data, a.Data) {
+		t.Errorf("input request was mutated; first part = %+v", req.Messages[0].Parts[0])
 	}
 }