[Fix] Recognize managed-role mentions of the Discord bot

[daniel-lxs]

Discord's mention autocomplete frequently resolves a bot by its managed role (same name as the bot). Such messages carry mention_roles + <@&role> content with an empty user-mention list, and the gateway classified every one as not-a-mention — the bot was silently unreachable for any user whose client picked the role variant. Found live while dogfooding: every @Roomotedev mention arrived and was dropped.

Fix

• The session learns the bot's managed role per guild from GUILD_CREATE role tags, with a lazy GET /guilds/{id}/roles fallback for resumed sessions (which never replay GUILD_CREATE)
• Role mentions are normalized at the gateway into a canonical user mention (content rewritten to <@bot>, schema-complete user entry injected) so downstream entry gating and mention stripping work unchanged
• New contract test: the normalized envelope must parse with the API's real discordGatewayEventSchema — the first injection attempt used a bare {id} and the API 400'd the events into the dead-letter stream (which is how [Improve] Harden the Discord gateway delivery and leadership paths #410's DLQ visibility surfaced it)
• The mock's managed role now carries its tags.bot_id so the harness can exercise this path

Validation

• 78/78 discord-gateway tests (3 new: normalization + schema contract, foreign-role ignored, unknown-role fallback)
• check-types 26/26, oxlint, format, knip green
• Verified live against real Discord end-to-end during dogfooding

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