Stabilize Agent session-isolation test on shared CI runners

[AnthonyRonning]

The desktop Agent session-isolation regression test times out after one second while waiting for an entire prompt to complete, including Goose session/database I/O. Research's version-only 3.4.4 PR hit this deadline on a shared Linux runner: an unrelated lifecycle holder must not block the first provider poll: Elapsed(()), with the rest of the suite passing.

Use a ten-second hang deadline for this test and document its purpose. The unrelated lifecycle mutex remains held until the prompt completes, and provider-call/terminal-message assertions stay intact. A real dependency on that mutex still prevents completion and fails the test. No application behavior or release versions change.

Validation:

• Focused regression test passed, followed by 20 consecutive exact-test repetitions.
• Normal pinned native pre-commit checks passed: 419 library and 3 CLI tests, with 2 existing ignored tests; Rust formatting and locked dependency checks passed.
• git diff --check passed.

Failure evidence: Rust CI on the version-bump PR. Kept separate from the isolated 3.4.4 bump.