fix(web): handle bot credit errors at source

[chrarnoldus]

Summary

• classify only AI SDK 402 responses carrying the Kilo gateway usage_limit_exceeded contract
• handle expected credit exhaustion in Kilo Bot instead of forwarding it to Sentry
• terminate callback continuations cleanly and post the gateway credit message to the user
• preserve Sentry reporting for provider 402s, malformed responses, network failures, and server errors

This is a source-level alternative to #5058 and avoids a global Sentry beforeSend filter.

Verification

• pnpm --filter web test -- --runInBand src/lib/ai-gateway/usage-limit-error.test.ts
• pnpm exec oxlint --config .oxlintrc.json apps/web/src/lib/ai-gateway/usage-limit-error.ts apps/web/src/lib/ai-gateway/usage-limit-error.test.ts apps/web/src/lib/bot/agent-runner.ts apps/web/src/lib/bot/run.ts apps/web/src/app/api/internal/bot-session-callback/[botRequestId]/route.ts
• pnpm --filter web typecheck
• pnpm format
• git diff --check

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed all 5 changed files: the new getKiloUsageLimitErrorMessage classifier matches the gateway's usageLimitExceededResponse 402 contract exactly (literal error_type + non-empty error.message, verified against llm-proxy-helpers.ts), the callback route's new catch correctly rethrows non-usage-limit errors so Sentry reporting is preserved for provider 402s and unexpected failures, and error propagation from runBotAgent through handleCompletedCallback is confirmed; unit tests cover the positive, wrong-status, malformed-body, and unrelated-error paths.

Files Reviewed (5 files)

• apps/web/src/lib/ai-gateway/usage-limit-error.ts
• apps/web/src/lib/ai-gateway/usage-limit-error.test.ts
• apps/web/src/lib/bot/agent-runner.ts
• apps/web/src/lib/bot/run.ts
• apps/web/src/app/api/internal/bot-session-callback/[botRequestId]/route.ts

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_{Reviewed by kimi-k3 · Input: 55.4K · Output: 14.5K · Cached: 741.9K}

_{Review guidance: REVIEW.md from base branch main}