fix: review findings — NaN threshold guard, percent-escape rejection in upstream model ids

- Segment: reject NaN thresholds (NaN fails every comparison, so it
  passed the [0,1] range check and reached the shim as the literal
  string "NaN"); ±Inf were already caught by the range comparisons,
  now covered by tests too.
- upstreamPath: reject '%' in model ids — %2F/%2E%2E percent-escapes
  decode back into path structure server-side, bypassing the literal
  /?#/.. rejection. Ids never legitimately contain '%'.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WWCQcYStWXBYUy5sZWnbLT

provider/llamaswap/upstream.go

@@ -16,12 +16,14 @@ import (
 //
 // Why reject rather than escape: same rationale as Unload — model ids
 // legitimately contain ":" but never path-structure characters, and escaping
-// would mask a config error instead of surfacing it.
+// would mask a config error instead of surfacing it. '%' is rejected too:
+// ids never legitimately carry percent-escapes, and %2F/%2E%2E would decode
+// back into path structure on the server side.
 func upstreamPath(model, rest string) (string, error) {
 	if strings.TrimSpace(model) == "" {
 		return "", fmt.Errorf("llama-swap: upstream call requires a model id")
 	}
-	if strings.ContainsAny(model, "/?#") || strings.Contains(model, "..") {
+	if strings.ContainsAny(model, "/?#%") || strings.Contains(model, "..") {
 		return "", fmt.Errorf("llama-swap: invalid model id %q for upstream call (contains a path separator)", model)
 	}
 	if !strings.HasPrefix(rest, "/") {