fix(python): report bound server port

[promiseeuler]

Related

• Closes Python Server(port=0) reports an unusable port-zero URL #12905

What

While using rerun-sdk==0.35.0, I found that rr.server.Server(host="127.0.0.1", port=0) binds an OS-assigned ephemeral port but returns rerun+http://127.0.0.1:0, which clients cannot use.

This change builds the Python server URL after startup from ServerHandle::connect_addr(), so it reports the actual bound port while preserving the existing connectable-host behavior for unspecified bind addresses. It also adds a regression test that verifies the reported port is nonzero and accepts a TCP connection.

Confidence: high. The Python binding was constructing the URL from the requested address before the server started; the server handle already exposes the resolved connect address after binding.

Validation:

• pixi run py-build
• pixi run py-fmt
• pixi run lint-rerun rerun_py/src/server.rs rerun_py/tests/unit/test_server.py
• rustup run stable rustfmt --edition 2024 --check rerun_py/src/server.rs
• pixi run uv run pytest -q rerun_py/tests/unit/test_server.py -k "port_zero_reports_bound_port or random_port" — 2 passed
• git diff --check

The complete test_server.py run reached 12 passing tests and 3 dataset-fixture failures because this shallow checkout contains Git LFS pointer files instead of the RRD fixtures (the loader reads vers rather than an RRF2 header).

Disclosure

I used an LLM-assisted workflow for this contribution. I manually reproduced the released behavior, reviewed the binding and server-handle paths, implemented the fix, and ran the validation listed above.

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