#19674: test: allow optional session key in subagent announce mock

Fix TS2345 in subagent-announce.format.e2e.test by allowing undefined sessionKey in chatHistoryMock. <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixes TypeScript compilation error TS2345 in test mock by making the `sessionKey` parameter optional in `chatHistoryMock`. The mock is called with potentially `undefined` sessionKey (`typed.params?.sessionKey` on line 71), so the parameter type needed to accept optional values to match actual usage. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - Single-line test-only change that fixes a legitimate TypeScript type error without affecting any production code or test behavior - No files require special attention _{Last reviewed commit: 365ac48} <!-- 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 -->