#12273: fix: prevent billing error false positive on bare '402' in chat content

## Summary - `ERROR_PATTERNS.billing` included `/\b402\b/` which matches any text containing the bare number '402' - Since `sanitizeUserFacingText()` runs on all outbound assistant text, normal chat content mentioning '402' (e.g. discussing x402 protocol, HTTP status codes) triggers a spurious billing warning - Replace `/\b402\b/` with `/\b402\s+payment/i` so the pattern only fires on actual '402 Payment Required' error responses - The existing `"payment required"` string pattern already covers the full error message Closes #12192 ## Test plan - [x] Existing tests for `isBillingErrorMessage` still pass (positive matches: "HTTP 402 Payment Required", "insufficient credits", etc.) - [x] New test verifies no false positive on chat content mentioning '402' - [x] `sanitizeUserFacingText` regression tests pass (10 tests) 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR narrows the billing-error detection regex in `src/agents/pi-embedded-helpers/errors.ts` to avoid false positives when assistant output contains the bare number `402` (since `sanitizeUserFacingText()` runs over normal assistant chat content). It updates `ERROR_PATTERNS.billing` from `/\b402\b/` to `/\b402\s+payment/i`, relying on explicit “Payment Required” wording (and the existing `"payment required"` string pattern) to identify true billing failures. A new unit test in `src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts` asserts that general chat text mentioning `402` no longer triggers billing warnings. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is narrowly scoped (one regex tweak) with a matching unit test addition to prevent regression; it reduces known false positives without affecting other billing indicators (e.g., "payment required", "insufficient credits"). No behavioral changes outside billing-error classification paths were introduced. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->