#23557: fix: catch session JSONL write failures instead of crashing

Closes #23556 ## Summary - Introduces a `safeAppend` helper in `installSessionToolResultGuard` that wraps `originalAppend` in a try/catch - Replaces all three bare `originalAppend` call sites with `safeAppend` - On write failure, logs `[session-guard] session write failed (<reason>); message dropped (file: <path>)` to stderr and returns gracefully instead of crashing ## Motivation Under Colima with virtiofs mounts, the session JSONL file can transiently become unwritable (`EACCES`). The existing code lets the thrown error propagate uncaught, which crashes the gateway with `exit(1)`. This is a disproportionate response to a non-fatal I/O condition — the active conversation can continue even if one history entry is lost. ## Test plan - [ ] New e2e tests in `session-tool-result-guard.e2e.test.ts` cover all three `safeAppend` call paths (flush, persisted, finalMessage) with a mock `originalAppend` that throws - [ ] Verify gateway stays running when `originalAppend` throws — error is logged, not propagated - [ ] Existing tests pass unchanged <!-- greptile_comment --> <h3>Greptile Summary</h3> Wraps session JSONL writes in try-catch via `safeAppend` helper to prevent gateway crashes from transient I/O errors (like `EACCES` under Colima virtiofs). All three `originalAppend` call sites now use `safeAppend`, which logs write failures to stderr and returns gracefully instead of propagating the exception. - Main change: introduces `safeAppend` wrapper (lines 153-168) that catches exceptions from `originalAppend` - Error handling: logs `[session-guard] session write failed (<reason>); message dropped (file: <path>)` and returns `undefined` - Test coverage: new tests verify all three call paths (flush, tool result, final message) with mocked write failures - Impact: conversation continues even if session history entry is lost due to I/O error <h3>Confidence Score: 4/5</h3> - This PR is safe to merge with one minor concern about type safety - The implementation correctly wraps all append calls with error handling and has comprehensive test coverage. The approach is sound - continuing the conversation when a single history write fails is more graceful than crashing. However, there's a type-unsafe cast (`undefined as unknown as ReturnType<typeof originalAppend>`) that could potentially cause issues if callers depend on the return value, though the existing code paths show `undefined` is already a valid return value in some scenarios. - No files require special attention - the implementation is straightforward and well-tested _{Last reviewed commit: 7b67c26} <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->