feat(msteams): opt-in canonical /api/messages Activity ingress (triton#247)

[jrosskopf]

Closes part of #247 (Activity Protocol adapter — the Microsoft Copilot/Teams ingress).

What

Generalizes triton-chat-msteams into triton's single, host-agnostic Microsoft Activity ingress (the "unify" decision from #247): the adapter now optionally serves the canonical Bot Framework path POST /api/messages in addition to the legacy /{name}/webhook. That is the endpoint an Azure Bot points at, and the path Teams, M365 Copilot Chat, WebChat and Copilot Studio channels all POST to. handle_webhook was already transport-generic (reads the Authorization header + raw body, not the route name), so one route reaches every Activity channel.

Design (post crew review)

• Opt-in, single-claimant. inbound.canonical_path (default off). The path is fixed and axum::Router::merge panics on an overlapping route, so a second claimant would crash the process — and its REST/MCP/A2A listeners — at startup. triton-bin now refuses a second claimant with a named error (exit 2), matching the existing NFR-S-4 guards in the same function.
• Auth unchanged / fail-closed. The canonical route reuses handle_webhook, so Bot Framework JWT verification (RS256, pinned kid, aud, serviceUrl allowlist) runs before the body is parsed. Forged bearers → 401, no dispatch.
• Adaptive Card stays 1.4. The transpiler emits no property above AC 1.3 (input_widget uses only label/isRequired), and a host capped below the declared version renders fallbackText instead of the card — a downgrade on exactly the non-Teams surfaces this ingress serves. Bump only when a 1.5-only feature is actually emitted. (An earlier 1.4→1.5 + fallbackText change was reverted after the crew review flagged the downgrade risk and a notification-preview leak.)

Scope reshaped by the spike

The Phase-0 spike found Copilot Studio's external-connect path (A2A JSON-RPC) flattens Adaptive Cards to text; the rich-card path is Activity Protocol over an Azure Bot channel → Teams (which this adapter already does). So the issue's expectReplies / dev-None-auth items — which target the A2A-orchestrator/skill model — are cut, not built as dead code.

Tests

• canonical_api_messages_path_dispatches — opted-in /api/messages dispatches identically to the legacy route.
• api_messages_enforces_auth — forged JWT on /api/messages → 401, no outbound reply.
• api_messages_absent_unless_opted_in — /api/messages → 404 by default; /msteams/webhook still works.

Full suite green: 33 msteams lib tests + 29 msteams integration tests; clippy clean on triton-chat-msteams + triton-bin.

Follow-ups (not in this PR)

• Consumer wiring: datazoo-agent-template opts in (TRITON_MSTEAMS_CANONICAL_PATH, default on) — lands with the vendor bump.
• Pre-auth audit rate-limiting on public webhook paths (crew F3) — pre-existing behaviour on /msteams/webhook; filing separately.

🤖 Generated with Claude Code

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