Fix aiohttp Socket Mode close lifecycle

[xero-aiescape]

Summary

Fixes the aiohttp Socket Mode shutdown race described in #1913.

• stop connect() once client shutdown begins, including when close lands during an in-flight failed attempt
• discard and close a WebSocket that completes after the client has already closed instead of installing new monitor/receiver tasks
• cancel and await the SDK-owned message processor, session monitor, and receiver tasks before closing the aiohttp ClientSession
• avoid cancelling or awaiting the current task when an owned reconnect task is itself coordinating close
• preserve normal ping_interval backoff, proxy, SSL, and live connection setup behavior

Testing

• ./scripts/run_tests.sh tests/slack_sdk_async/socket_mode/test_aiohttp.py — 10 passed
• ./scripts/run_tests.sh tests/slack_sdk_async/socket_mode/ — 18 passed before the final added close-success race; the final file and full CI-equivalent suite cover that addition
• CI-equivalent Python 3.14 unit command from .github/workflows/ci-build.yml with AWS host credentials/region removed — 984 passed, 5 skipped
• ./scripts/run_mypy.sh --no-install — 106 source files clean
• focused Black/Flake8 and Semgrep Python checks — clean
• ./scripts/build_pypi_package.sh — sdist/wheel built and twine check passed

Red-before-green lifecycle coverage verifies:

• an already-closed real aiohttp client returns instead of retrying a closed ClientSession
• close during a failed connection attempt terminates the orphan-capable task
• a connection that succeeds after close is discarded and its WebSocket is closed
• live connection failures still wait for ping_interval
• close() does not return until all owned task cleanup settles and before the client session closes
• an owned task coordinating close does not cancel or await itself

Category

• slack_sdk.socket_mode (Socket Mode client)
• tests/integration_tests (Automated tests for this library)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run the repository validation commands in a repo-local virtual environment after making the changes.

Fixes #1913

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @xero-aiescape to sign the Salesforce Inc. Contributor License Agreement.

[mwbrooks]

Hi @xero-aiescape, thanks for the PR! 🙇🏻

Before we can review your pull request, can you please sign the CLA? ✏️

Thanks!