Port `onOptionsLoad` handler for dynamic select dropdown population

[patrick-chinchill]

Upstream feature missing in Python SDK.

What's missing

Upstream Vercel Chat TS exposes onOptionsLoad() for registering handlers that dynamically populate Select / RadioSelect dropdown options when a user opens a modal — returning { value, label }[] live from the handler.

Upstream references

• vercel-chat/packages/chat/src/types.ts:655 — processOptionsLoad method on ChatInstance
• vercel-chat/packages/chat/src/chat.ts:610-629 — onOptionsLoad() decorator with action-ID matching
• vercel-chat/packages/chat/src/chat.ts:911 — processOptionsLoad dispatch
• Types: OptionsLoadEvent, OptionsLoadHandler, OptionsLoadPattern

What we need to port

1. Types: OptionsLoadEvent, OptionsLoadHandler in src/chat_sdk/types.py
2. Chat.on_options_load() decorator with action-ID matching (explicit matches first, then catch-all)
3. Chat.process_options_load() entry point for adapters
4. ChatInstance protocol: add process_options_load method
5. Adapter integration: Slack, Teams dispatch options_load events when the platform requests option population

Impact

• Required for modals that need server-side dynamic dropdowns (e.g., "pick from my Jira projects", "pick from my Linear issues")
• chinchill-api doesn't currently use it, so not blocking

Acceptance

• Types + handler registration + dispatch
• Slack adapter emits events when block_suggestion payload arrives
• Teams adapter emits events when config/fetch arrives
• Integration tests on recorded fixtures
• Entry in docs/UPSTREAM_SYNC.md non-parity table cleared