config: resolve the draft worker's config per runner, not on a copy

The v2 spec workers got a published `ServerArgs` copy carrying two values: the
target's context length and `--speculative-draft-load-format`. Neither is a
process-wide config change — each is consumed by exactly one constructor — so
the copy, the publish switch around the draft build, and the replay of the
target's resolved overrides onto it all go away, and the values travel to the
runner that owns them:

- **Context length.** `TpModelWorker` already takes it (`context_length=None`
  keeps `server_args.context_length`); the four v2 draft workers and
  `build_draft_tp_worker` pass the target's, which every one of them has in
  scope as `target_worker` / `target_model_config`.
- **Load format.** `ModelRunner._draft_load_format()` resolves it for a draft
  runner and `build_load_config` takes it, so the `LoadConfig` is per-runner.
  Model code also reads it off the bag while it builds — Inkling replaces
  per-element noise in its shared-expert scales under dummy loading — so the
  load is wrapped in a scoped bag override that puts the target's value back.
- `skip_tokenizer_init` was on the copy for nobody: `TpModelWorker` already
  short-circuits the tokenizer for a draft worker (`or self.is_draft_worker`).

`PrefillCudaGraphRunner._max_addressable_prefix_len` capped the prefix by
`server_args.context_length`, which the copy used to carry for the draft; it now
reads the runner's own `model_config.context_len`. That is also more accurate for
the target, whose `--context-length` may be unset while the resolved context is
shorter than the token table.

What stays a variant is the dflash/dspark path's attention backend: backend
selection reads it off the config object the draft runner holds, and the
resolved gate has to survive the variant's publish. `draft_server_args_overrides`
now carries only those fields and says why.

python/sglang/srt/runtime_context.py

@@ -636,8 +636,13 @@ class _ConfigBag:
 
     @contextmanager
     def override(self, **kwargs):
-        """Scoped, transactional test-only override of this bag's own leaves
-        (keys validated before any write; restored on exit)."""
+        """Scoped, transactional override of this bag's own leaves (keys
+        validated before any write; restored on exit).
+
+        For a window where one runner's value differs from the process's — a
+        draft model loading under ``--speculative-draft-load-format`` while the
+        target keeps ``--load-format`` — and for tests forcing a code path.
+        A permanent change goes through ``get_context().override``."""
         fields = object.__getattribute__(self, "_fields")
         unknown = set(kwargs) - set(fields)
         if unknown: