Pin LiveKit server Docker image

[WilliamK112]

Summary

• Pin the embedded LiveKit server Docker image to livekit/livekit-server:v1.13.1 instead of latest.
• Reuse the runtime image constant in the LiveKit integration test pre-pull step so tests exercise the same image reference.

Fixes #27.

Testing

• uv run python -m py_compile server-runtime\xr_media_hub\transport\livekit\_docker.py tests\test_integration_livekit.py
• uv tool run --from ruff==0.15.16 ruff check server-runtime\xr_media_hub\transport\livekit\_docker.py tests\test_integration_livekit.py
• git diff --check origin/main..HEAD
• Confirmed no livekit/livekit-server:latest references remain under server-runtime or tests
• uv run pytest -rs -v test_integration_livekit.py from the tests project collected the module but skipped it because Docker is not on PATH in this local Windows environment

Rebased onto current origin/main (ec4169aa) and force-pushed with lease.

[nvddr]

The CI failure here is unrelated to this PR's changes. Root cause: pipecat-ai>=0.0.46 in the dependency tree allows resolving to v0.0.79, which transitively pulls numba v0.53.1 → llvmlite v0.36.0, which only supports Python <3.10. Main CI has been passing because it hits a warm uv cache with a newer resolution; fork PRs build without cache access, get a fresh resolve, and land on the old version.

PR #260 fixes the underlying floor (pipecat-ai>=0.0.46 → >=1.0 along with a few other stale floors). Once that lands, this PR should pass CI.