feat(v2): add WithPromptCaching() request option

Introduces an opt-in RequestOption that callers can pass to enable
automatic prompt-caching markers. The option populates a cacheConfig
on requestConfig but has no effect yet — plumbing through to
provider.Request and on to the Anthropic provider lands in subsequent
commits.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/request_test.go

@@ -135,3 +135,22 @@ func TestBuildProviderRequest_EmptyConfig(t *testing.T) {
 		t.Errorf("expected no tools, got %d", len(req.Tools))
 	}
 }
+
+func TestWithPromptCaching(t *testing.T) {
+	cfg := &requestConfig{}
+	WithPromptCaching()(cfg)
+	if cfg.cacheConfig == nil {
+		t.Fatal("expected cacheConfig to be set after WithPromptCaching()")
+	}
+	if !cfg.cacheConfig.enabled {
+		t.Error("expected cacheConfig.enabled to be true")
+	}
+}
+
+func TestWithoutPromptCaching(t *testing.T) {
+	cfg := &requestConfig{}
+	// No option applied
+	if cfg.cacheConfig != nil {
+		t.Error("expected cacheConfig to be nil when option not applied")
+	}
+}