fix(discord): properly route slash event handling in threads

[simpolism]

What does this PR do?

Native Discord slash event handling doesn't work in threads -- it gets routed to the group-level session instead. This means commands like /usage and /reset get routed incorrectly, the former returning incorrect usage data, the latter failing to reset the thread's session. It works when forced into a text string rather than using Discord native commands.

This PR ensures the correct source gets constructed for thread-based slash events.

Related Issue

Fixes #2011.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• gateway/platforms/discord.py -- added chat_type = "thread" handling for source construction.
• tests/gateway/test_discord_slash_commands.py -- added test cases for _build_slash_event handling.

How to Test

1. Start a thread via Discord Gateway and chat a bit.
2. Confirm Discord native /usage command returns expected result.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass -- confirmed all tests/gateway/ tests pass, but full test suite seemed hung
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: Ubuntu 24.04

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

For New Skills

• This skill is broadly useful to most users (if bundled) — see Contributing Guide
• SKILL.md follows the standard format (frontmatter, trigger conditions, steps, pitfalls)
• No external dependencies that aren't already available (prefer stdlib, curl, existing Hermes tools)
• I've tested the skill end-to-end: hermes --toolsets skills -q "Use the X skill to do Y"

Screenshots / Logs

[teknium1]

Merged via PR #2460. Cherry-picked with authorship preserved. Clean fix + good tests. Thanks!