fix: await async MCP header providers

[he-yufeng]

Link to Issue or Description of Change

Closes: #6090

Problem:

header_provider is called from async MCP execution paths, but async providers were not awaited. Passing an async provider therefore left a coroutine object in the header merge path, which fails with TypeError: 'coroutine' object is not iterable and leaves the coroutine unawaited.

Solution:

Await awaitable header_provider results before merging headers in both MCP paths:

• McpToolset._execute_with_session, used while creating sessions for get_tools() / resources.
• McpTool._run_async_impl, used when an individual MCP tool call creates a session.

The existing sync provider behavior is unchanged.

Testing Plan

Unit Tests:

• Added async header_provider coverage for McpToolset.get_tools().
• Added async header_provider coverage for McpTool._run_async_impl().
• Passed locally:
  • PYTHONPATH=src python -m pytest tests/unittests/tools/mcp_tool/test_mcp_tool.py tests/unittests/tools/mcp_tool/test_mcp_toolset.py -q

Additional checks:

• python -m py_compile src/google/adk/tools/mcp_tool/mcp_tool.py src/google/adk/tools/mcp_tool/mcp_toolset.py
• python -m pyink --check src/google/adk/tools/mcp_tool/mcp_tool.py src/google/adk/tools/mcp_tool/mcp_toolset.py tests/unittests/tools/mcp_tool/test_mcp_tool.py tests/unittests/tools/mcp_tool/test_mcp_toolset.py
• python -m ruff check src/google/adk/tools/mcp_tool/mcp_tool.py src/google/adk/tools/mcp_tool/mcp_toolset.py
• git diff --check

Manual E2E was not run; the changed behavior is covered by the MCP unit tests above.