#15109: fix: distinguish transient API errors from billing errors

Fixes #15055 ## Summary Fix error message handling to distinguish between transient API failures (rate limits, timeouts) and actual billing issues. ## Problem Previously, all API failures showed the same billing error message, making it impossible for users to distinguish between: - Genuine billing issues (insufficient credits) - Temporary errors (rate limiting, timeouts) ## Changes - Check `isRateLimitErrorMessage()` before `isBillingErrorMessage()` - Check `isTimeoutErrorMessage()` before `isBillingErrorMessage()` - Add tests for rate limit (429) and timeout error handling - Ensure transient errors show appropriate retry messages ## Test Results - ✅ All existing tests pass (16/16 in formatassistanterrortext.test.ts) - ✅ New tests added for rate limit and timeout scenarios - ✅ Full test suite: 6235 tests passed ## AI-Assisted Development This PR was developed with assistance from Claude Opus 4.6. The code has been reviewed and tested to ensure correctness. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com> <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates `formatAssistantErrorText` to classify rate-limit and timeout errors before billing errors, and adds tests to ensure transient errors don’t display the billing/credits messaging. Within the wider codebase, assistant text is also passed through `sanitizeUserFacingText(..., { errorContext: true })` in multiple user-facing pipelines (assistant text extraction and reply normalization). That sanitizer still collapses rate-limit errors into the generic “temporarily overloaded” message in its `ERROR_PREFIX_RE` rewrite path, which can negate the PR’s goal on those surfaces. <h3>Confidence Score: 4/5</h3> - Mostly safe to merge, but there is one remaining user-facing classification inconsistency for rate limits. - The change to `formatAssistantErrorText` is small and well-covered by targeted tests, but another sanitizer used in user-facing flows still rewrites rate-limit errors to the overloaded message, which conflicts with the stated behavior change and can cause incorrect messaging in some surfaces. - src/agents/pi-embedded-helpers/errors.ts _{Last reviewed commit: 7e1df79} <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->