feat(agents): add merge_agent_headers utility

Mirrors merge_mcp_headers from the MCP server utils.
Dynamic headers come first; static (admin-configured) headers overlay and win on conflict.

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

litellm/proxy/agent_endpoints/utils.py

@@ -0,0 +1,27 @@
+"""Utility helpers for A2A agent endpoints."""
+
+from typing import Dict, Mapping, Optional
+
+
+def merge_agent_headers(
+    *,
+    dynamic_headers: Optional[Mapping[str, str]] = None,
+    static_headers: Optional[Mapping[str, str]] = None,
+) -> Optional[Dict[str, str]]:
+    """Merge outbound HTTP headers for A2A agent calls.
+
+    Merge rules:
+    - Start with ``dynamic_headers`` (values extracted from the incoming client request).
+    - Overlay ``static_headers`` (admin-configured per agent).
+
+    If both contain the same key, ``static_headers`` wins.
+    """
+    merged: Dict[str, str] = {}
+
+    if dynamic_headers:
+        merged.update({str(k): str(v) for k, v in dynamic_headers.items()})
+
+    if static_headers:
+        merged.update({str(k): str(v) for k, v in static_headers.items()})
+
+    return merged or None