feat(chat-platform): one Hazel bot for every Maple region - #2244

🟢 Confidence 10/10 · safe to merge. One helper derives the region for both the install link and the redemption, and new tests pin both plus the gateway handshake.
_{no findings · tests covered · risk medium}

Adds an optional per-deployment gateway region so several Maple instances can share one Hazel bot, threading it through the install link, the callback URI and the token exchange, with optional connector config keys so a missing region still counts as configured. The three files changed since the last review are consistent with each other and with the callers; safe to merge once the rollout steps land.

• hazelRedirectUri puts ?hazel_region= on the install link and the redemption
• MAPLE_HAZEL_REGION declared as an optional key on the install and gateway halves
• isConnectorConfigured and chat-bot's resolveConnectorConfig ignore optional keys

Before merge

• Env var · Set MAPLE_HAZEL_REGION=eu in the EU api and chat-bot env; Hazel's regional install support must be deployed first · packages/chat-platform/src/connectors/hazel/api.ts:42
• Manual · Register EU's callback on the bot as …/oauth/chat/hazel/callback?hazel_region=eu — the PR description still says ?region=eu, which Hazel will not match

_{c52178c · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}