fix: clean Mattermost progress breadcrumbs

[admin-prodesign]

Summary\n- add Mattermost post deletion support so cleanup_progress can remove temporary progress/status bubbles\n- sanitize noisy provider retry/fallback errors for Mattermost like Telegram\n- cover Mattermost cleanup and provider-noise behavior with focused tests\n\n## Tests\n- python -m pytest tests/gateway/test_telegram_noise_filter.py::test_mattermost_status_suppresses_retry_noise tests/gateway/test_telegram_noise_filter.py::test_mattermost_status_sanitizes_raw_provider_errors tests/gateway/test_telegram_noise_filter.py::test_mattermost_final_response_sanitizes_raw_provider_errors tests/gateway/test_mattermost.py::TestMattermostSend::test_delete_message_calls_api_delete tests/gateway/test_mattermost.py::TestMattermostSend::test_delete_message_failure_returns_false -q -o 'addopts='\n

[teknium1]

Thanks for the focused Mattermost cleanup work. The deletion support addresses a current gap: gateway/run.py:17233-17238 disables cleanup_progress unless an adapter overrides delete_message, and plugins/platforms/mattermost/adapter.py currently has no such override.

Problems

• The gateway/run.py sanitizer hunk is now superseded. Current main sanitizes all human-facing gateway platforms through gateway/run.py:94-107 and gateway/run.py:414-457; Mattermost is already covered, with parametrized coverage in tests/gateway/test_telegram_noise_filter.py:13-75. Preserve that broader behavior rather than restoring a Telegram/Mattermost-only gate.
• The timeline-linked fix(mattermost): classify API errors, escalate fatals, lock single-instance, surface audio/slash attachments #35645 identifies an overlapping Mattermost edit_message rewrite, so its adapter changes and this deletion support need manual reconciliation if both are salvaged.

Suggested changes

• Retain the Mattermost DELETE implementation and its focused tests, adapted to the current adapter structure; drop the superseded sanitizer/test portion.

Automated hermes-sweeper review.

[admin-prodesign]

Addressed the sweeper review in 42556f7:

• rebased the PR onto current main
• dropped the superseded gateway/run.py sanitizer changes and noise-filter tests
• retained only Mattermost post deletion support and its focused tests
• kept the adapter change scoped to the current Mattermost structure (not the overlapping fix(mattermost): classify API errors, escalate fatals, lock single-instance, surface audio/slash attachments #35645 rewrite)

Verification:

• 272 passed for tests/gateway/test_mattermost.py and tests/gateway/test_telegram_noise_filter.py
• Ruff clean on the changed adapter/test files
• git diff --check clean