config: retire ServerArgs.override in favour of derive()

`ServerArgs.override(source, **fields)` was the last way to change a resolved
`ServerArgs` in place. Every remaining call-site was one of two things, and
neither wanted an in-place write:

- **A config for someone else.** A draft worker's context length, an encode
  worker's device, the compile script's watchdog, the client's port pick, a test
  fixture's backends. These already deepcopied first — the write was on the copy.
- **A launcher-stage resolution.** `resolve_auto_parsers` detected the chat
  template's parsers and wrote them back, to be inherited by the schedulers it
  spawns.

Both are "one config becomes another", so `derive(source, **fields)` returns the
variant and leaves the receiver — and any bags projected from it — untouched. It
deliberately is not `dataclasses.replace`: resolution does not re-run, because
the values being set are decided after it, from inputs it never had. Provenance
and the resolvable-field stash work as before, on the copy.

`resolve_auto_parsers` now computes the parsers and returns the config to launch
with; the detection helpers stop taking a config to mutate. `HiMambaRadixCache`
re-applied a HiCache layout normalization `__post_init__` already performs (the
same duplicate removed from `UnifiedRadixCache` in ebb1c88d23) and just goes.

With no in-place mutation left, `ServerArgs.__setattr__` raising after
resolution *is* the guarantee, so the textual writer ratchet retires and
`test_server_args_derive.py` pins the contract instead: the receiver survives
deriving, the published instance still refuses assignment, and deriving does not
publish. `SGLANG_STRICT_CONFIG_MUTATION` was already unused — the guard has been
unconditional since the mutation sweep — and goes with it.

The detection tests drop their `SimpleNamespace` stand-in for a real
`ServerArgs`; the test kit and the MLA chunk-metadata fixture publish a derived
variant instead of writing the runner's published config.

python/sglang/lang/backend/runtime_endpoint.py

@@ -384,13 +384,15 @@ class Runtime:
         from sglang.srt.server_args import ServerArgs
         from sglang.srt.utils.network import is_port_available
 
-        self.server_args = ServerArgs(*args, log_level=log_level, **kwargs)
-
-        # Pre-allocate ports
-        for port in range(self.server_args.port, 40000):
+        # Pre-allocate a port before building the config, so the config is born
+        # with the port this runtime will serve on.
+        requested_port = kwargs.pop(
+            "port", ServerArgs.__dataclass_fields__["port"].default
+        )
+        for port in range(requested_port, 40000):
             if is_port_available(port):
                 break
-        self.server_args.override("runtime_endpoint.port_alloc", port=port)
+        self.server_args = ServerArgs(*args, log_level=log_level, port=port, **kwargs)
 
         self.url = self.server_args.url()
         self.generate_url = self.url + "/generate"