fix: use working-directory to resolve adk_pr_triaging_agent module path

[qizwiz]

Problem

The `agent-triage-pull-request` job fails with:

```
ModuleNotFoundError: No module named 'adk_pr_triaging_agent'
```

The module lives at `contributing/samples/adk_team/adk_pr_triaging_agent/`. Setting `PYTHONPATH` to a relative path is fragile and doesn't reliably add the directory to `sys.path`.

Fix

Replace the `PYTHONPATH` env var with `working-directory: contributing/samples/adk_team` on the triaging step. When Python is invoked with `-m`, it automatically adds the current working directory to `sys.path`, making `adk_pr_triaging_agent` importable without needing `PYTHONPATH` at all.

This is the same approach recommended by Copilot's analysis of the failure.

Testing plan

• The fix is a one-line workflow change (swap `PYTHONPATH` env var → `working-directory`); no Python source changes.
• Verified locally: `python -m adk_pr_triaging_agent.main` invoked from `contributing/samples/adk_team/` resolves the module correctly without `PYTHONPATH`.
• CI evidence: the triaging bot successfully ran on PR refactor: centralise JSON parsing via json_utils.safe_json_loads #5858 (posted a review comment), confirming the workflow executes end-to-end after this fix.

[adk-bot]

Response from ADK Triaging Agent

Hello @qizwiz, thank you for submitting this pull request to fix the triaging script module path issue!

To help our reviewers process this change more efficiently, please ensure it aligns with our Contribution Guidelines. Currently, the following sections are missing from your PR description:

1. Testing Plan: Please add a Testing Plan section in your PR description indicating how the changes were or should be verified (for example, pointing to the successful GitHub Actions run of this PR).
2. Verification Evidence: Please include brief console logs or confirmation showing that the agent-triage-pull-request run succeeds after your fix is applied.

We appreciate your contribution and help in improving our CI/CD workflows!