CI: bind test_sync_mode_agent listener on an ephemeral port (#1965)

## What?

Bind `test_sync_mode_agent[enable_listen=True]`'s listener on an
ephemeral port (`listen_port=0`) instead of the fixed default `8888`.

## Why?

- The four `enable_listen=True` cases all bind `8888`, so they (and
parallel CI jobs sharing a node) collide: `Failed to bind metadata
listener socket`.
- Sample: [nixl-ci-gpu
#2829](https://nbuprod.blsm.nvidia.com/nbu-swx-nixl-main/job/nixl-ci-gpu/2829/)
- 4 failed, 20 passed.

## How?

- `listen_port=0` lets the OS pick a free port (already supported). The
test only asserts `syncMode`, so the port is irrelevant.


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## Summary by CodeRabbit

* **Bug Fixes**
* Improved concurrent test runs by allowing the operating system to
select an available network port, reducing potential port collisions.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->