fix(skills): isolate per-toolset failures in SkillToolset additional-tools resolution

[hawktang]

Summary

SkillToolset._resolve_additional_tools_from_state resolves a skill's declared adk_additional_tools by gathering get_tools_with_prefix() from every provided toolset in a single asyncio.gather(...) with no per-toolset error isolation:

ts_results = await asyncio.gather(*(
    ts.get_tools_with_prefix(readonly_context)
    for ts in self._provided_toolsets
))

If any one toolset raises while listing its tools — most commonly an McpToolset whose remote MCP server is restarting / unreachable / returns 503 — the entire gather propagates and no additional tools resolve for the activated skill, including unrelated individual function tools that were already available with zero I/O.

In practice, a single optional remote MCP being briefly down breaks every skill that declares adk_additional_tools — surfacing as an internal error even for skills that never reference the failing MCP.

Fix

• Pass return_exceptions=True to the gather, then skip (with a logger.warning) any toolset whose listing failed.
• Re-raise asyncio.CancelledError so cooperative cancellation is not swallowed.

A failing/unreachable toolset now contributes no tools and recovers automatically on the next invocation (tool listing is cached per invocation_id).

Test

Adds test_skill_toolset_resolution_isolates_failing_toolset: a toolset whose get_tools_with_prefix raises is placed in the additional_tools pool alongside a healthy tool. After activating a skill, the healthy tool and core skill tools still resolve, the failing toolset contributes nothing, and a warning is logged. Verified red before the change, green after.

Notes

• Reproduces on 2.1.0, 2.3.0, and current main.
• The existing experimental _MCP_GRACEFUL_ERROR_HANDLING guards the MCP session layer, but not this aggregation layer, so the failure still propagated.
• Open design question: should a failure always degrade (this PR — right for optional toolsets), or hard-error only when the failing toolset would have provided a tool the active skill actually declares in adk_additional_tools? Happy to adjust.

🤖 Generated with Claude Code

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[adk-bot]

Response from ADK Triaging Agent

Hello @hawktang, thank you for creating this PR!

It looks like there are a couple of things missing based on our contribution guidelines:

1. Contributor License Agreement (CLA): The CLA check is currently failing. Please sign the Google CLA at https://cla.developers.google.com/ to proceed.
2. Testing Plan: Please include a formal Testing Plan section in your PR description. In particular, we'd love to see a summary of passed pytest results showing that the newly added test passes locally.

Once these are addressed, we will triage and label your PR. Thank you!