#13318: fix(agents): prevent sanitizeUserFacingText from rewriting conversational context overflow mentions

## Fix The function `sanitizeUserFacingText` was incorrectly rewriting valid assistant responses that happened to mention "context overflow" (e.g., explaining why a request failed previously). This PR adds `isLikelyAssistantContent` heuristic to detect structured responses (markdown, paragraphs, conversational markers) and skip the rewrite unless the text is explicitly a raw error payload. Fixes #13310. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR changes `sanitizeUserFacingText` to avoid rewriting normal assistant-facing prose that happens to mention “context overflow”, by introducing an `isLikelyAssistantContent` heuristic (length/paragraphs/markdown/conversational starts) and only applying the “raw error payload” formatting when the input still looks like an API/HTTP error. It also adds a new Vitest file to cover the intended behavior: raw context-overflow errors should still be rewritten to the user-friendly message, while conversational/markdown explanations containing the phrase should pass through unchanged. <h3>Confidence Score: 3/5</h3> - This PR should not be merged until the new test import is fixed and the sanitizer’s `<final>`-stripping behavior is made consistent in the new heuristic branch. - The core logic change is small and well-targeted, but the added test appears to import from a module path that doesn’t exist in-repo, and the new early-return path formats errors from the unstripped input, which can regress existing sanitization guarantees. - src/agents/pi-embedded-helpers.context-overflow.test.ts, src/agents/pi-embedded-helpers/errors.ts <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->