fix: MCPToolset does not include authentication information during initialization and tool listing

[chyok]

PR Description

Summary

This PR adds a redundant get_header method to MCPToolset, with the exact same functionality as the existing get_header in mcp_tool.

This is a minimal-change, fast-fix approach to address the reported authentication issue, enabling us to resolve the bug in stages:

• Stage 1 (this PR): Introduce get_header in MCPToolset to quickly unblock users and fix the bug.
• Stage 2 (future work): Refactor by extracting get_header into a shared utility to avoid duplication or refactor the module.

With this PR, the MCPToolset will now correctly handle authentication headers during MCP initialization and tool listing.

Linked Issue

This PR addresses #2168

Testing Plan

Unit Tests

• All unit tests for the new get_header method have been added under tests/unittests/tools/mcp_tool/test_mcp_toolset.py

• All tests were run using pytest and passed successfully.

Manual End-to-End (E2E) Test

• Verified the MCPToolset integration using the ADK runner.
• Confirmed that after this fix, the MCP tool can be initialized and used without authentication errors.

Additional Notes

• This is an interim fix to minimize code changes and risk.
• In follow-up PRs, we will refactor and deduplicate the get_header method by moving it into a shared utility module.

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[chyok]

Hi @seanzhou1023 , would it be acceptable to allow a small amount of duplicated code, or do we still prefer to extract it into a separate function?
I can try to refactor the logic for credentials to the header, but it might introduce larger changes, which could make the review process more difficult.