fix(proxy): scope model availability by account

[Draculabo]

Summary

• keep image-model 404 failures scoped to the failing account
• preserve sibling image availability and non-image capability failures
• update the BaseProxyService regression test for the current shared retry-policy dependency

Validation

• 22 focused proxy tests
• TypeScript type-check
• targeted ESLint

[codereviewbot-ai]

Summary of Changes

• Image Model 404 Penalty Isolation: Updated BaseProxyService.applyUpstreamPenalty to skip calling markModelUnrequestable for image models (-image), ensuring image 404 responses are scoped per-account rather than disabling the model across all accounts.
• Model Availability Clearance Refinement: Refactored ModelAvailabilityService.clearModel to remove side-effect clearing of sibling image models when clearing a specific model's availability status.
• Test Coverage: Added comprehensive unit test coverage in base-proxy-model-not-found-scope.test.ts and updated proxy-model-availability-store.test.ts.

LGTM!

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[codereviewbot-ai]

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