#10792: Agents: avoid context overflow false positives

Fixes a false-positive sanitizer rewrite where normal text that *mentions* "Context overflow" gets replaced with the generic overflow error message. What changed: - Only rewrite "context overflow" when the text looks like a standalone error line (error prefix, HTTP status prefix, or starts with "Context overflow:" / "Context overflow-" / "Context overflow—"). - Added tests for both the false-positive case and real overflow error lines. Tests: - pnpm check - pnpm vitest run src/agents/pi-embedded-helpers.sanitizeuserfacingtext.test.ts Fixes: #8847 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Tightens `sanitizeUserFacingText()` so “context overflow” is only rewritten when the input resembles a standalone error line (error prefix, HTTP status prefix, or `Context overflow:`/dash forms). - Adds Vitest coverage to ensure normal conversational mentions of “Context overflow” are not clobbered. - Adds Vitest coverage verifying real standalone overflow error lines still get rewritten to the generic user-facing overflow guidance. - Change is localized to PI embedded helpers error sanitization; other error formatting paths remain unchanged. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is narrowly scoped to sanitization heuristics, uses existing prefix regexes, and includes targeted tests covering the regression and expected rewrite cases; no broader behavior changes were observed in related error formatting paths. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **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 -->