[WRONG BRANCH] fix(auth): map compact credential-substitution failures to 401

[luvs01]

Motivation

• The native /v1/responses/compact path threaded bearer admission through but did not mirror the same fail-closed handling for missing stored-main credential substitution, causing an internal 500 instead of a 401 when substitution was unavailable.

Description

• Map CodexMainSubstitutionUnavailableError to a fail-closed 401 authentication_error in the compact auth resolution path so compact requests behave the same as the main Responses path. (Change in src/server/responses/compact.ts.)
• Add an end-to-end regression that posts to /v1/responses/compact with a bearer admission and verifies a missing stored main credential returns 401 and performs no upstream I/O. (New test in tests/codex-envkey-admission-substitution.test.ts.)
• Small test helper added to drive the compact endpoint and integrated into the existing [Bug]: restore Codex env_key bearer admission and direct-mode OpenAI credential substitution lidge-jun/opencodex#1686 test matrix.

Testing

• bun test tests/codex-envkey-admission-substitution.test.ts (focused): passed.
• bun x tsc --noEmit (typecheck): passed.
• bun run privacy:scan: passed.
• A repository-wide bun run test / full-suite run was observed but not relied on here due to long-running unrelated tests in this environment; the focused regression and typecheck were used as the primary verification.

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Codex Task

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📥 Commits

Reviewing files that changed from the base of the PR and between 2bd271f and ebc109f.

📒 Files selected for processing (2)

• src/server/responses/compact.ts
• tests/codex-envkey-admission-substitution.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.