feat(web): version-gated cutover to conversationId on POST /v1/messages + GET /v1/events (LUM-1890 Phase 2)

[vellum-apollo-bot]

Summary

Phase 2 of the LUM-1890 wire-format migration. The web client now sends the canonical conversationId field to daemons that support it, and falls back to the legacy conversationKey for daemons that don't.

Phase 1 (#31922, merged at 0fbf523aa9) made the daemon bilingual — POST /v1/messages and GET /v1/events accept either conversationKey (legacy external-key lookup) or conversationId (canonical internal-id lookup). This PR cuts the web client over to the canonical field with a version gate.

The version gate adopts the lib/backwards-compat/ pattern established in #31932 — gates live in one directory, reuse the shared @/utils/semver.js utilities, and document their MIN_VERSION per feature area.

Why the gate

The web client and the macOS daemon ship on independent release cadences. Users update the daemon by installing a fresh macOS app build; the web is whatever is currently deployed at app.vellum.ai. The web is usually ahead of the local daemon. When we introduce a new wire field, the web must keep speaking the old field name to daemons that don't know the new one yet.

Cutover threshold: daemon >= 0.8.5. Phase 1 will ride in 0.8.5 (current main is 0.8.4).

What changed

New gate — apps/web/src/lib/backwards-compat/conversation-id-wire-field.ts

const MIN_VERSION = "0.8.5";

export type ConversationIdWireField = "conversationId" | "conversationKey";

export function pickConversationIdWireField(): ConversationIdWireField {
  const version = useAssistantIdentityStore.getState().version;
  if (!version) return "conversationKey";
  const parsed = parseSemver(version);
  const min = parseSemver(MIN_VERSION);
  if (!parsed || !min) return "conversationKey";
  return compareParsed({ ...parsed, pre: null }, min) >= 0
    ? "conversationId"
    : "conversationKey";
}

Reads the version snapshot via useAssistantIdentityStore.getState() rather than the use.version() hook selector, so it's safe to call from the non-hook async paths (postChatMessage, subscribeChatEvents). Semver parsing reuses @/utils/semver.js so behavior matches the existing useAssistantSupports hook used by flag-query-freshness: pre-release suffixes count as the full patch (0.8.5-rc.1 → supported), v prefix stripped, unparseable / missing versions fall back to the legacy field.

Outbound wire sites

Two — both now do [pickConversationIdWireField()]: conversationId to pick the right wire-field name on each call:

1. postChatMessage (messages.ts) — POST /v1/messages body.
2. subscribeChatEvents (stream.ts) — GET /v1/events query.

Tests

• conversation-id-wire-field.test.ts (new, 5 tests): unknown → conversationKey, 0.8.4 / 0.7.0 → conversationKey, 0.8.5 / 0.8.6 / 0.9.0 / 1.0.0 → conversationId, RC builds (0.8.5-rc.1, 0.8.5-beta) → conversationId, unparseable → conversationKey. Exhaustive semver-edge tests already live in utils.test.ts.
• post-chat-message.test.ts — adds postChatMessage wire-field bilingual cutover describe (4 tests) covering the matrix at the integration level (asserting against the actual outbound request body). Adds clearIdentity() in beforeEach/afterEach so the existing block defaults to the legacy path.
• stream.test.ts — adds 3 bilingual tests mirroring the same matrix for the SSE subscribe query param. Broadens the existing omits conversationKey query … test to confirm both wire fields are absent.

Out of scope (this PR)

• event-parser.ts reads data.conversationKey from inbound events; the daemon still emits this in deep-link metadata.
• routes.tsx URL-redirect translating legacy ?conversationKey= URLs in-app to ?conversationId= is a URL semantics shim, not a wire-format concern.
• Inspector / event-bus / type-definition references to conversationKey are inbound or client-internal.

Out of scope (later phases)

• Phase 3: macOS Swift client cutover (6 files).
• Phase 4: daemon drops legacy conversationKey from the wire. Ships after macOS catches up. The version gate above means newer web clients gracefully downshift against older daemons; the breaking direction (older web talking to a Phase-4 daemon) we control via the deploy gate.

Validation

• bun run typecheck — clean
• bun run lint (touched files) — clean
• bun test src/domains/chat/api/ src/lib/backwards-compat/ — 232/232 pass

Pending follow-up from Phase 1

After approving #31922, two editorial comments were left on conversation-management-routes.ts. Both queued in notes/pending-followups.md; they'll ride on the next PR that touches that file. This PR only modifies apps/web/, so they don't fold in here.

Refs LUM-1890.

[linear]

LUM-1890