fix(slack): sanitize generated session IDs

[anneheartrecord]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: SlackRunner error when agent uses managed session service. ValueError: Invalid session_id 'C0BARCEJJ1K-1784622590.251599': must match ^[A-Za-z0-9_-]+$. #6435

2. Or, if no issue exists, describe the change:

Problem:
SlackRunner builds ADK session IDs from Slack channel/thread identifiers. Slack timestamps include ., for example C0BARCEJJ1K-1784622590.251599, but managed session services validate session IDs against ^[A-Za-z0-9_-]+$.

Solution:
Sanitize Slack-derived session ID parts before passing them to Runner.run_async, replacing characters outside the managed-session-safe set with _. Slack API calls still use the original Slack thread_ts, so Slack threading behavior is unchanged.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed locally:

uv run --with pytest --with pytest-asyncio --with slack-bolt --with aiohttp pytest tests/unittests/integrations/slack/test_slack_runner.py -q
# 4 passed, 4 warnings

Also ran:

uv run --with ruff ruff check src/google/adk/integrations/slack/slack_runner.py tests/unittests/integrations/slack/test_slack_runner.py
uv run --with isort isort --check-only src/google/adk/integrations/slack/slack_runner.py tests/unittests/integrations/slack/test_slack_runner.py
uv run --with pyink pyink --check src/google/adk/integrations/slack/slack_runner.py tests/unittests/integrations/slack/test_slack_runner.py

Manual End-to-End (E2E) Tests:

Not run. The unit test covers the reported Slack thread timestamp shape and verifies that Runner.run_async receives a managed-session-safe ID while Slack replies still use the original thread_ts.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

The focused regression uses thread_ts="1234567890.123456" and expects session_id="C67890-1234567890_123456".

[rohityan]

Hi @sasha-gitg , can you please review this. LGTM