[Integration DoD][MW-06] Add dedicated subscription OAuth e2e flow coverage

[github-actions]

This issue is auto-generated from INTEGRATION_DOD_MAP.md by the nightly DoD gap sync workflow.

Gap

• ID: MW-06
• Owner (area-owner): API

Acceptance Criteria

• E2E tests cover OAuth start, exchange, credential persistence, and expected failure paths.

Verification Commands

bunx vitest run --config vitest.e2e.config.ts test/subscription-auth.e2e.test.ts

Risk

• Login regressions are found late.

Source

• INTEGRATION_DOD_MAP.md:462

[hellopleasures]

Fix submitted in PR #766.

Bug fix: Anthropic OAuth exchange failure now cleans up state._anthropicFlow — previously leaked on failure, blocking subsequent /start calls. The OpenAI handler already cleaned up in its catch block; Anthropic was inconsistent.

3 new tests (22→25):

• Anthropic flow cleanup verification after failed exchange (proves the bug fix)
• startAnthropicLogin throw → 500 error path (was untested)
• startCodexLogin throw → 500 error path (was untested)