fix(ai-llm-proxy): enforce server-side Origin check (403 on mismatch)

[dj4oC]

Problem

ai-llm-proxy validates the oCIS bearer token and sets CORS response headers, but has no server-side Origin check. CORS headers are advisory and enforced only by the browser, so they do not stop a non-browser or cross-site client from reaching the LLM. The repo security requirement is explicit: any LLM-calling path "must perform an explicit server-side origin check and return 403 for unexpected origins — CORS headers alone are browser-enforced and insufficient."

This came up while reviewing the new AI extensions (#475, #476, #477), which all correctly route through this proxy and rely on it being the single enforcement point — but the enforcement was missing.

Fix

Add an authoritative server-side Origin gate in handleRequest, before token validation and the LLM call:

• Origin header present but not matching the origin derived from OCIS_URL → 403.
• No Origin header → allowed through to token validation. A cross-site browser request always carries Origin, so a missing one cannot be a cross-site browser attack, and the mandatory oCIS bearer token still guards it.
• No-op when OCIS_URL is unset (the server already refuses to start without it outside tests).

The expected origin is computed once from OCIS_URL via new URL(OCIS_URL).origin, so it compares correctly against the scheme/host/port Origin header.

Testing

• New isOriginAllowed unit tests: matching origin, cross origin, scheme mismatch, port mismatch, missing header, unconfigured.
• pnpm --filter ai-llm-proxy test:unit → 25 passed.
• pnpm --filter ai-llm-proxy build + lint clean.

Risk

Low. Genuine calls from the oCIS web extensions are same-origin POSTs that always carry Origin: <OCIS_URL origin>, so they are unaffected. Only requests from a different (or unexpected) origin are newly rejected — which is the intent.

[kw-security]

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