fixup(adk): enforce empty-dict contract when parent context_id missing + annotate test helper

Addresses Copilot review feedback on kagent-dev#1927.

- _build_lineage_headers now hard-returns {} when the local session id is
  unresolvable. Without this guard the function would emit a root-only
  header forwarded from inbound state, contradicting the docstring
  contract "no session id → no lineage headers" and matching the
  existing unit-test assertion.

- _make_tool's header_provider parameter gains Callable type annotation
  matching the production signature.

Signed-off-by: Lukas <1775218+Prefix@users.noreply.github.com>

Author: Prefix

python/packages/kagent-adk/src/kagent/adk/_remote_a2a_tool.py

@@ -285,18 +285,19 @@ def _build_lineage_headers(self, tool_context: ToolContext) -> dict[str, str]:
                 if isinstance(hdrs, dict):
                     inbound_headers = hdrs
 
+        if not parent_context_id:
+            return {}
+
         root_context_id = (
             inbound_headers.get(ROOT_CONTEXT_ID_HEADER)
             or inbound_headers.get(PARENT_CONTEXT_ID_HEADER)
             or parent_context_id
         )
 
-        headers: dict[str, str] = {}
-        if parent_context_id:
-            headers[PARENT_CONTEXT_ID_HEADER] = str(parent_context_id)
-        if root_context_id:
-            headers[ROOT_CONTEXT_ID_HEADER] = str(root_context_id)
-        return headers
+        return {
+            PARENT_CONTEXT_ID_HEADER: str(parent_context_id),
+            ROOT_CONTEXT_ID_HEADER: str(root_context_id),
+        }
 
     async def run_async(self, *, args: dict[str, Any], tool_context: ToolContext) -> Any:
         """Execute the remote agent tool.

python/packages/kagent-adk/tests/unittests/test_remote_a2a_tool.py

@@ -1,6 +1,6 @@
 """Tests for KAgentRemoteA2ATool."""
 
-from typing import Any, AsyncIterator
+from typing import Any, AsyncIterator, Callable
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import httpx
@@ -121,7 +121,7 @@ async def _async_yield(*items) -> AsyncIterator:
 def _make_tool(
     *,
     httpx_client: httpx.AsyncClient | None = None,
-    header_provider=None,
+    header_provider: Callable[[Any], dict[str, str]] | None = None,
 ) -> KAgentRemoteA2ATool:
     return KAgentRemoteA2ATool(
         name="k8s_agent",