fix: surface Anthropic OAuth rate limits

[guillaumegay13]

Summary

• Preserve Anthropic OAuth token-exchange rate-limit failures as HTTP 429 instead of collapsing them to a generic bad-request error.
• Surface the backend exchange error message in the Anthropic subscription UI so users see the rate-limit retry guidance.
• Add focused backend and frontend regression coverage.

Validation

• npm run build --workspace=packages/shared
• npm test --workspace=packages/backend -- src/routing/oauth/anthropic/anthropic-oauth.service.spec.ts src/routing/oauth/anthropic/anthropic-oauth.controller.spec.ts
• npm test --workspace=packages/frontend -- AnthropicOAuthDetailView.test.tsx
• git diff --check

Notes

Split out of #1950 so the MPS refactor stays focused on model parameter schema and scoped saved values.

---

Summary by cubic

Preserves Anthropic OAuth token-exchange rate limits as HTTP 429 and surfaces the provider’s guidance in the UI. This helps users retry correctly instead of seeing a generic expired-code error.

• Bug Fixes
  • Map both HTTP 429 and provider rate_limit_error bodies to 429 via AnthropicOauthExchangeError.
  • Controller forwards the service error status instead of collapsing to 400.
  • UI shows the backend error message in AnthropicOAuthDetailView, and falls back to a generic message for non-Error failures.
  • Added tests for 429 handling, message rendering, and the non-Error fallback path.

^{Written for commit a764018. Summary will update on new commits. Review in cubic}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.25%. Comparing base (5a35436) to head (a764018).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1958      +/-   ##
==========================================
- Coverage   99.31%   99.25%   -0.07%     
==========================================
  Files         170      169       -1     
  Lines       14903    15337     +434     
  Branches     5820     5993     +173     
==========================================
+ Hits        14801    15222     +421     
- Misses        101      113      +12     
- Partials        1        2       +1     

Flag	Coverage Δ
frontend	99.26% <100.00%> (-0.05%)	⬇️
shared	98.75% <ø> (-1.25%)	⬇️

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[cubic-dev-ai]

No issues found across 7 files

_{Re-trigger cubic}

[codecov]

Bundle Report

Changes will increase total bundle size by 11.9kB (0.61%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
manifest-frontend-esm	1.96MB	11.9kB (0.61%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: manifest-frontend-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
assets/index-*.css	3.45kB	187.79kB	1.87%
assets/Routing-*.js	8.24kB	117.82kB	7.52% ⚠️
assets/ProviderSelectModal-*.js	32 bytes	111.93kB	0.03%
assets/index-*.js	47 bytes	56.73kB	0.08%
assets/Overview-*.js	-3 bytes	32.81kB	-0.01%
assets/overview-*.js	-1.66kB	25.37kB	-6.14%
assets/MessageLog-*.js	2 bytes	10.47kB	0.02%
assets/Select-*.js (New)	3.68kB	3.68kB	100.0% 🚀
assets/api-*.js	41 bytes	3.48kB	1.19%
assets/AuthBadge-*.js (Deleted)	-1.93kB	0 bytes	-100.0% 🗑️

Files in assets/ProviderSelectModal-*.js:

• ./src/components/AnthropicOAuthDetailView.tsx → Total Size: 13.22kB