fix(discord): add typing indicator toggle

[DevZenPro]

Summary

• add a DISCORD_TYPING environment toggle for Discord typing indicators
• bridge discord.typing from config.yaml to DISCORD_TYPING, matching the existing auto_thread / reactions pattern
• add tests for disabling typing via env and for the config-to-env bridge

Intent / context

Discord typing indicators are cosmetic, but they hit the Discord REST route:

POST /channels/{channel_id}/typing

In live gateway usage, repeated typing refreshes during long-running agent responses can contribute to Discord 429/resource-rate-limit behavior, especially when combined with auto-thread creation and reaction add/remove calls. Once Discord starts returning 429s, retries/reconnects can make the bot appear silent or unstable even though message handling itself is fine.

This change lets operators disable only the typing indicator traffic while keeping Discord message replies working normally:

DISCORD_TYPING=false

or:

discord:
  typing: false

The default remains unchanged: typing indicators stay enabled unless explicitly disabled with false, 0, or no.

Test plan

• python -m pytest tests/e2e/test_discord_typing_config.py tests/gateway/test_discord_slash_commands.py::test_discord_typing_config_bridge -q -o 'addopts='

[teknium1]

Superseded by #55394 (merged to main, commit 05ac167). That PR adds a generic per-platform typing_indicator flag at the shared _keep_typing spawn site, which covers Discord — the same key now works for every platform, so set typing_indicator: false under your discord platform config. Thanks for raising this first (May 8) — credited in the merged PR.