feat(mattermost): add rich markdown support for formatted messages (#59401)

[webtecnica]

Add rich markdown support for Mattermost platform. Adds "props": {"format": "markdown"} to all post payloads. Closes #59401

[teknium1]

Thanks for covering the Mattermost gateway and its standalone sender consistently.

Problems

• Required CI is currently red: tests/tools/test_send_message_missing_platforms.py:115 still asserts the old exact standalone payload, while the changed _standalone_send payload now includes props.format. The failed required slice reports TestSendMattermost.test_success.
• This PR says it closes #59401, but that issue requests an opt-in mattermost.rich_markdown/extended_markdown configuration and a Mattermost-flavored Markdown transformer. Current format_message() already preserves standard Markdown at plugins/platforms/mattermost/adapter.py:476-484; this diff only adds REST payload properties and does not add the requested config or transformation.

Suggested changes

• Update the exact-payload test and add coverage for the changed adapter send paths.
• Either rescope the PR away from closing #59401 or implement the issue's requested opt-in behavior.

Automated hermes-sweeper review.

[teknium1]

Please update the standalone sender's exact-payload assertion at tests/tools/test_send_message_missing_platforms.py:115. Required CI currently fails because it still expects only channel_id and message; include the intended props contract in that test.