#2878 tools: give the bench's start branch back the `port` local it reads (#2872)

Beschreibung

`./bench.sh` with no `--url` dies before it starts the model:

```
  File "tools/strata_bench.py", line 990, in main
    say(f"Starting {cfg_path.name} on port {port} (loading takes 1-3 minutes; the PC may be slow meanwhile) ...")
NameError: name 'port' is not defined
```

69fe4055 (#2330) replaced

```python
port = a.port or int(cfg.get("port", 8080))
base = f"http://127.0.0.1:{port}"
```

with `base = server_base(cfg.get("host"), a.port or int(cfg.get("port", 8080)))`, which removed the only
assignment of the local `port` - but the branch that starts its own server still reads it three times: the
"Port N is used by something that is not Strata" message, the "Starting ... on port N" message, and
`start_server(cfg_path, port, ...)`.  `--url` skips the branch, so it never tripped there.

This keeps the #2330 behaviour - the poll still goes to `server_base(cfg host, port)`, so a `0.0.0.0` bind is
polled on loopback - and changes nothing else in the branch.

### What I changed

- `tools/strata_bench.py`: restore the local, keep `server_base()`.
- `tools/test_bench_host.py`: a `StartsItsOwnServer` class that drives `main()` into the branch with
  `start_server` and `wait_ready` stubbed out.  The existing tests here only called `server_base()`
  directly, which is why the missing local got through.

It checks the port the server is given, the address polled (`--port` overriding the config, and loopback for
the config's `0.0.0.0`), and the foreign-service-on-the-port case.

### How I checked

On main today the new tests fail with the reported traceback at `strata_bench.py:990`; on this branch all 8
tests in `tools/test_bench_host.py` pass (`python -m unittest tools.test_bench_host`).  No GPU is needed.

Fixes #2872.  #2864 is the same report and should close with it.

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