fix(genai): tighten containsGenerateContent path matching

Require a delimiter (: or /) before the method name in
containsGenerateContent to avoid potential false positives on
hypothetical endpoints like deleteGenerateContentCache.

Also document the single-candidate limitation in
postprocessStreamingResults.

Author: AbhiPrasad

trace/contrib/genai/generatecontent.go

@@ -205,7 +205,11 @@ func (gt *generateContentTracer) postprocessStreamingResults(allResults []map[st
 		return nil
 	}
 
-	// Aggregate text parts from all candidates across chunks
+	// Aggregate text parts from all candidates across chunks.
+	// NOTE: We only aggregate candidate index 0 from each chunk. If the API
+	// ever returns multiple candidate indices in a streaming response, their
+	// content will be silently dropped. The non-streaming path passes through
+	// all candidates as-is.
 	var textParts []string
 	var finishReason any
 	var role string

trace/contrib/genai/tracegenai.go

@@ -152,10 +152,13 @@ func genaiRouter(cfg *config, path string) internal.MiddlewareTracer {
 	return nil
 }
 
-// containsGenerateContent checks if the path is for a generateContent or streamGenerateContent endpoint
+// containsGenerateContent checks if the path is for a generateContent or streamGenerateContent endpoint.
+// We require a delimiter (: or /) before the method name to avoid matching unrelated endpoints.
 func containsGenerateContent(path string) bool {
-	return strings.Contains(path, "generateContent") ||
-		strings.Contains(path, "GenerateContent")
+	return strings.Contains(path, ":generateContent") ||
+		strings.Contains(path, "/generateContent") ||
+		strings.Contains(path, ":streamGenerateContent") ||
+		strings.Contains(path, "/streamGenerateContent")
 }
 
 // isStreamingPath checks if the path is for the streaming endpoint