feat(discord): add message edit and delete admin actions

[StatPan]

Summary

• add discord_admin.edit_message(channel_id, message_id, content)
• add discord_admin.delete_message(channel_id, message_id)
• wire both through the existing Discord action manifest, schema, handler defaults, required-parameter validation, and 403 hints
• add tests for action calls, registry handler content propagation, admin/core schema split, validation, and 403 enrichment

Part of #1559.

Test Plan

• python3 -m pytest tests/tools/test_discord_tool.py -q -o addopts=