fix: exception type

[superlevure]

Tentative fix for https://github.com/nats-io/nats.py/pull/934/changes#r3823522044

[fallenmi]

Reviewed with Codex at exact head dcf8fcd.

I independently exercised all three modified timeout exits against the exact base and head: 90 deterministic base executions raised asyncio.TimeoutError, while 90 exact-head executions raised FetchTimeoutError. The latter remains catchable through the documented nats.errors.TimeoutError base class, restoring the pre-2.15 public exception contract across the initial no-wait timeout and both exhausted-deadline branches.

The touched module also compiles cleanly, the patch passes git diff --check, and all 27 live checks are green. The head and zero-feedback state were unchanged immediately before submission.