feat(tools): add Discord integration (#2913)

[mishrapravin114]

Description

Adds Discord integration enabling agents to list guilds, list channels, send messages, and get recent messages via Discord API v10. Includes 4 MCP tools with full credential store and health check support.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #2913
Related: #2805 (parent integration tools issue)

Changes Made

• Added discord_tool package with 4 MCP tools for Discord API v10 integration
• Implemented _DiscordClient class using httpx for HTTP operations
• Added DiscordHealthChecker for bot token validation before agent execution
• Added comprehensive test suite with 20 tests covering all operations and error cases
• Registered Discord tools in main tools/init.py
• Created DISCORD_CREDENTIALS credential spec with credential store support
• Created detailed README.md with setup, usage examples, and API reference

Tools implemented:

• discord_list_guilds - List guilds (servers) the bot is a member of
• discord_list_channels - List channels for a guild
• discord_send_message - Send a message to a channel
• discord_get_messages - Get recent messages from a channel

Testing

• Unit tests pass (cd tools && pytest tests/tools/test_discord_tool.py tests/test_health_checks.py -v)
• 20 Discord tests + 44 health check tests passed
• Lint passes (ruff check on added files)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes