#16307: fix: surface billing/auth FailoverErrors as user-friendly messages

Root cause: When `FailoverError` with billing or auth reason is thrown after all models fail, the catch block in `runAgentTurnWithFallback` falls through to the generic "Agent failed before reply" handler instead of producing a clean, actionable error message. Fix: Add explicit `isFailoverError` check before the generic fallback, routing billing errors to `formatBillingErrorMessage()` and auth errors to a provider-specific authentication failure message. Test: 5 cases covering billing/auth with and without provider name, verifying clean user-facing messages and absence of generic "Agent failed before reply" text. <!-- greptile_comment --> <h3>Greptile Summary</h3> Improved error handling for billing and authentication failures by surfacing `FailoverError` instances with specific, user-friendly messages instead of falling through to the generic "Agent failed before reply" handler. Key changes: - Added explicit `isFailoverError` check in `agent-runner-execution.ts:548-567` before the generic fallback handler - Routes billing errors to `formatBillingErrorMessage()` and auth errors to `formatAuthErrorMessage()` with provider-specific context when available - Updated `pi-embedded-runner/run.ts:881` to use `formatAuthErrorMessage()` instead of generic "LLM request unauthorized." text - Added comprehensive test coverage with 5 test cases covering both billing and auth scenarios with/without provider names - Tests verify clean messages and absence of generic error text <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation is straightforward, well-tested, and follows existing patterns. The change adds specific error handling for FailoverError cases before falling through to generic handlers, which improves user experience without altering any core logic. Test coverage validates all code paths including with/without provider names for both billing and auth scenarios. - No files require special attention _{Last reviewed commit: 244a1f7} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->