Fix Discord allow_from config bridge for gateway auth

[Que0x]

Summary

config.yaml-based Discord allow_from values were being loaded into PlatformConfig.extra, but they were never bridged to DISCORD_ALLOWED_USERS. Since both the gateway auth layer and the Discord adapter enforce authorization from env-backed allowlists, the config-defined allowlist was effectively ignored.

This PR completes that bridge. discord.allow_from and platforms.discord.extra.allow_from now populate DISCORD_ALLOWED_USERS while preserving existing env-overrides-first behavior. That keeps the current auth model intact and makes the documented config path actually enforce access control.

Changes

• Bridge discord.allow_from to DISCORD_ALLOWED_USERS
• Bridge platforms.discord.extra.allow_from to DISCORD_ALLOWED_USERS
• Preserve env precedence when DISCORD_ALLOWED_USERS is already set
• Add regression coverage for both config shapes

Validation

• pytest -o addopts='' -p no:timeout tests/gateway/test_config.py -q -k "discord_allow_from or discord_thread_require_mention or discord_history_backfill or discord_channel_prompts"
4 passed
• pytest -o addopts='' -p no:timeout tests/gateway/test_discord_channel_controls.py tests/gateway/test_discord_bot_auth_bypass.py tests/gateway/test_discord_slash_auth.py -q
55 passed

[teknium1]

Merged via #35329 (#35329). Your commit was cherry-picked onto current main with authorship preserved (6d2727e). Combined with #34565 since both touched gateway/config.py and would have conflicted separately. Thanks!