feat(channel): add Mattermost channel

[longkeyy]

Summary

• add a built-in Mattermost channel plugin using Mattermost REST and WebSocket APIs
• validate server URLs and credentials, load bot identity, and reconnect WebSocket sessions with bounded timeouts/backoff
• support channel allowlists, self-message filtering, threaded replies, pairing, media/tool relay, and in-place streamed reply updates
• register Mattermost in plugin types, routes, defaults, and channel configuration

Configuration

• credential: accessToken
• settings: serverUrl, allowedChannelIds, replyInThread, ignoreSelfMessages

Safety And Reliability

• rejects non-HTTP(S) server URLs
• preserves stored credentials when the frontend saves settings without replacing the token
• ignores messages from the bot when configured
• filters allowed channel IDs before dispatch
• covers WebSocket TLS, timeout, close, reconnect, and streamed-message update behavior

Related

• Frontend PR: feat(channel): add Mattermost settings AionUi#3390
• Related issue: [Feature]: Add Mattermost remote channel support AionUi#3494

Verification

• rebased onto AionCore origin/main at fe448e02
• just push --force-with-lease longkeyy feat/mattermost-channel passed migration immutability, formatting, clippy, workspace compilation, and all 9344 nextest tests (52 skipped by configuration)
• cargo check -p aionui-channel --features mattermost passed
• cargo test -p aionui-channel mattermost passed
• release AionCore is injected into a macOS arm64 AionUi package and runtime-verified after the rebase

Delivery Dependency

After this PR merges, publish an AionCore release newer than v0.1.72 and update the frontend aioncoreVersion pin. The current public v0.1.72 artifact predates Mattermost even though the crate version on this branch remains 0.1.72.

[longkeyy]

Rebased onto current main and ready for review. The PR body now includes configuration, safety/reliability behavior, full local verification, frontend dependency, and the required release/pin follow-up. @kaizhou-lab @tcp404, could you please review when available?

[longkeyy]

Rebased onto current main (fe448e02) and force-updated the branch. The full local gate passed: migration immutability, formatting, clippy, workspace compilation, and 9344/9344 nextest tests (52 skipped by configuration). The PR body now reflects the current v0.1.72 release dependency and frontend pin follow-up. @kaizhou-lab @tcp404, this is ready for review again.