feat(api): wire bifrost sidecar fast-path into /v1/chat/completions and /v1/messages with native fallback

[rqzbeh]

Summary

This PR connects the supervised Go Bifrost sidecar (@maximhq/bifrost) directly into OmniRoute's primary high-traffic ingress endpoints (POST /v1/chat/completions and POST /v1/messages) based on release/v3.8.51.

• Zero-Copy Fast-Path: When Bifrost is active and healthy on loopback (127.0.0.1:8080), eligible requests bypass the heavy V8 handler stack and stream tokens directly through Go's net/http connection pool.
• Graceful Auto-Fallback: If Bifrost is offline, times out, or returns 5xx errors, requests automatically trip failure cooldowns and transparently fall through to native TypeScript handleChat() with X-Routing-Fallback headers attached.
• Shared Subsystem: Consolidates Bifrost routing, circuit breaker cooldowns, and client dispatching into src/shared/services/bifrost/, deduplicating logic across the relay proxy and primary routes.
• Test Suite: Added comprehensive unit and integration test suites covering unary dispatch, SSE streaming finalization, failure cooldowns, and fallback mechanics.

Verification

• npm run typecheck:core: Passed (0 errors).
• npm run check:route-guard-membership: Passed.
• npm run check:deps: Passed (0 new dependencies).
• node --import tsx/esm --test tests/unit/services/bifrost/*.test.ts tests/unit/api/v1/*bifrost*.test.ts: 22/22 tests passed (100% green).

⚠️ base-red inherited: #13866

[diegosouzapw]

This one's solid — real fetch-based dispatch with proper timeout, fallback to the native
handler on any 5xx/network error, and a cooldown so a downed sidecar doesn't get hammered.
Ran your 4 new/changed test files live against this branch's head: 12/12 pass. Two things
before merge: (1) this branch shares ~19 duplicated "fix(security)"/"fix(ci)" commits with
your other two PRs (#12967, #12962) that touch errorPathRedaction.ts — which has since
diverged on the release tip with a different fix — so a clean rebase dropping those is needed;
(2) dispatchToBifrost passes the sidecar's response straight through on any status < 500,
bypassing sanitizeErrorMessage() for 4xx bodies — worth confirming the bifrost binary never
puts internal paths/stack traces in an error body, or routing that through the same
sanitization as native errors.

[rqzbeh]

1. Rebased cleanly onto release/v3.8.51 (4d1282be31), dropping all diverged commits from main and leaving open-sse/utils/errorPathRedaction.ts untouched so the diff contains only the 10 Bifrost domain files.
2. In src/shared/services/bifrost/bifrostClient.ts, intercepted sidecar 4xx responses to run them through sanitizeErrorMessage() and buildErrorBody() per Hard Rule fix(ui): fix Select dropdown dark theme inconsistency #12 rather than passing them through verbatim, preventing path/credential leaks from third-party binary error payloads. Verified with unit test bifrostClient: dispatchToBifrost sanitizes a 4xx sidecar body instead of passing it through.

[diegosouzapw]

Re-homed to release/v3.8.52: v3.8.51 entered its release freeze, so the branch now belongs to the release captain and development continues on the next cycle. Nothing is wrong with this PR — it just needed a live base. No action needed from you; CI will re-run against the new base.