feat(channels): add Mattermost channel and harden gateway integration

[2niuhe]

📝 Description

This PR adds Mattermost channel support and hardens runtime stability for long-running gateway
sessions.

Included changes

• register Mattermost channel factory and wire it into gateway startup
• add channels.mattermost initialization path with required URL/token checks
• implement Mattermost channel runtime (websocket consume, send/edit/typing/media)
• add websocket ping/pong keepalive with reconnect backoff reset after successful connect
• add manager init regression tests for Mattermost enabled/invalid configs
• extend config schema/env mappings and required module dependencies

Why

• Fixes gateway startup failure when Mattermost is configured but channel init path was missing.
• Improves recovery from half-open websocket connections that could cause silent non-response after
  multi-turn chats.
• Prevents silent media-send failures by checking post status codes explicitly.

🗣️ Type of Change

• [] 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

• [Feature]Support Mattermost as a native PicoClaw chat channel #1587

📚 Technical Context (Skip for Docs)

• Reference URL:
  • https://api.mattermost.com/#tag/websocket
  • https://api.mattermost.com/#tag/posts
  • https://api.mattermost.com/#tag/users
• Reasoning:
  • Channel manager previously lacked Mattermost init branch, causing no channels enabled.
  • WebSocket read loops can stall on half-open connections; keepalive ping/pong forces health
    detection and reconnect.
  • Reconnect backoff should reset after a successful connection to avoid prolonged recovery after
    transient failures.

🧪 Test Environment

• Hardware: x86_64 PC
• OS: Linux (local dev environment)
• Model/Provider: Local existing provider configuration (unchanged by this PR)
• Channels: Mattermost

📸 Evidence (Optional)

Details

make check passed

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

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