config: spell out the one dynamic config read the census could not see

`_is_dsa_active` asked `getattr(server_args, "_is_dsa_model_arch", False)`, and
that name has never existed on `ServerArgs` -- it arrived as a placeholder with
the CP strategy abstractions (#27313), so the getattr default has always decided
the predicate. A dynamic read of a name nothing sets is the one shape the config
census cannot follow, and it looked like a live decision while being dead.

Spelled as the constant it evaluates to, with the placeholder written down: what
it should ask (whether this process runs a DSA model arch) is the CP path's
call, and its only consumer, `ContextParallelStrategy.per_layer_attn_cp_comm`,
has no readers yet.

That was the sole entry in the read ratchet's `_INERT_DYNAMIC_READS`, so the
exemption list is gone with it -- there is no way to exempt a read from the
baselines any more, which is the invariant worth having. The `counted()`
indirection it existed for goes too (verified the three shapes it guarded still
report: direct, `getattr`, and an attribute-parked alias).

python/sglang/srt/server_args.py

@@ -3527,6 +3527,9 @@ class ServerArgs:
     ] = None
 
     def __post_init__(self):
+        self._run_resolution_pipeline()
+
+    def _run_resolution_pipeline(self):
         """
         Orchestrates the handling of various server arguments, ensuring proper configuration and validation.