fix(git-info): stop stale polling after session replacement

[627150795]

Problem

After rapid session replacement, overlapping session_start handlers could leave polling fibers using an invalidated Pi context. The stale context was then accessed every five seconds and logged a background defect indefinitely.

Value

Stop stale polling without affecting the newest session's refresh loop, and explicitly interrupt polling during session shutdown.

Approach

• Bind each polling fiber to the session generation and context that created it.
• Ignore superseded session_start handlers after their previous-fiber interruption completes.
• Interrupt the tracked polling fiber during shutdown.
• Add a regression test covering rapid session starts, stale-context access, and continued polling on the newest context.

Validation

• bun run check
• node --test --experimental-strip-types tests/extensions/git-info/index.test.ts tests/extensions/git-info/refresh-coordinator.test.ts

The full suite still has one pre-existing Windows failure in tests/extensions/git-info/process.test.ts: a command exiting with code 7 is reported as -1.

Impact

No public API changes. The change is limited to git-info background polling lifecycle and its regression coverage.