#20262: fix: detect Venice 402 insufficient USD/Diem balance as billing failover [AI-assisted]

## Summary\nThis fixes a failover gap where Venice returns errors like:\n\n`402 "Insufficient USD or Diem balance to complete request"`\n\nThose messages were not classified as billing/failover errors, so model fallback cascades could stop early.\n\n### Changes\n- Expand billing regex to recognize generic quoted/bracketed leading 402 payloads.\n- Add explicit billing phrase match for `insufficient usd or diem balance`.\n- Add regression test coverage for the Venice 402 string.\n\n## Why\nOpenClaw fallback logic depends on failover classification. If this 402 shape is missed, fallback does not advance and users see raw provider errors.\n\n## Testing\nAI-assisted PR: yes (Codex/OpenClaw).\n\nDegree of testing: **lightly tested + targeted**\n\nRan locally:\n- `pnpm vitest run --config vitest.e2e.config.ts src/agents/pi-embedded-helpers.isbillingerrormessage.e2e.test.ts` ✅ (32/32 passed)\n- `pnpm lint` ✅\n- `pnpm tsgo` ✅\n- `pnpm build` ✅\n\n## Prompt / Session context\nIssue was identified from a real main-session transcript where fallback stalled after Venice returned the 402 string above. This patch is the minimal classification fix + regression test. <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR closes a failover classification gap where Venice returns `402 "Insufficient USD or Diem balance to complete request"` errors that were not recognized as billing errors. Without this fix, the model fallback cascade stalls and users see raw provider error messages. - Adds a new regex alternation `^\s*402\s*["'{[]` to the billing pattern to match 402 responses prefixed with quoted/bracketed payloads (e.g., Venice's format). The `^` anchor and structured-delimiter character class avoid false positives. - Adds an explicit `"insufficient usd or diem balance"` string pattern, which is needed because the existing `"insufficient balance"` pattern requires a contiguous substring match and does not match the Venice phrasing. - Adds the Venice 402 string to two test sections for regression coverage. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it is a minimal, well-targeted pattern expansion with regression tests and no risk of false positives. - The changes are small and focused: two additions to pattern-matching logic (one regex alternation, one string literal) plus corresponding test coverage. The regex uses a `^` anchor and structured-delimiter character class that prevent false positives, verified against the existing false-positive test suite. No control flow, API, or architectural changes are introduced. - No files require special attention. _{Last reviewed commit: ea39070} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->