fix: honor specific-tier fallbacks in proxy path

[Nicolas-Formenton]

Summary

• propagate fallback_models from specificity resolution
• prefer those fallbacks in the proxy when a specificity-routed primary fails
• add regression coverage for both resolve and proxy behavior

Why
Specific-tier fallbacks were persisted and configurable, but the runtime proxy path ignored them and only consulted general tier fallbacks.

What changed

1. packages/backend/src/routing/dto/resolve-response.ts

• add optional fallback_models?: string[] | null

2. packages/backend/src/routing/resolve/resolve.service.ts

• include assignment.fallback_models ?? null in resolveSpecificity(...)

3. packages/backend/src/routing/proxy/proxy.service.ts

• prefer resolved.fallback_models before general tier fallbacks

4. tests

• packages/backend/src/routing/resolve.service.spec.ts
  • assert specificity resolution propagates fallback models
• packages/backend/src/routing/proxy/__tests__/proxy.service.spec.ts
  • assert a specificity-routed request uses its own fallback chain even when the general tier fallback list is null

Test plan

• npm run build --workspace=packages/shared
• npm test --workspace=packages/backend -- --runInBand src/routing/resolve.service.spec.ts src/routing/proxy/__tests__/proxy.service.spec.ts

Additional note
To run the targeted tests in this environment, I also had to fix a pre-existing strict-typing compile issue in packages/backend/src/routing/routing-core/tier.service.ts by annotating the TIERS.map(...) callback parameter as Tier. If you'd prefer, I can split that into a separate cleanup PR.

Closes #1586

---

Summary by cubic

Ensure specificity-routed requests use their own fallback chain when the primary model fails. Propagates per-assignment fallback_models through resolve and prefers them in the proxy, fixing incorrect tier fallback behavior.

• Bug Fixes
  • Expose fallback_models on ResolveResponse and propagate from resolveSpecificity.
  • Proxy now prefers resolved.fallback_models before tier-level fallbacks.
  • Add regression tests for resolve and proxy fallback behavior.
  • Fix strict typing in tier.service.ts (TIERS.map callback typed as Tier).

^{Written for commit 4f91ccd. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.92%. Comparing base (67f8852) to head (4f91ccd).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1587   +/-   ##
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  Coverage   97.92%   97.92%           
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  Files         118      118           
  Lines        8675     8675           
  Branches     3265     3265           
=======================================
  Hits         8495     8495           
  Misses        178      178           
  Partials        2        2           

Flag	Coverage Δ
frontend	97.91% <ø> (ø)
shared	100.00% <ø> (ø)

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