test: recover the config-namespace-migration deferrals (#33171)

The module-skipped tests injected config by faking get_server_args (a
SimpleNamespace stand-in patched onto the module) or by writing fields
onto a ServerArgs instance post-publish — both invisible to the namespace
accessors the production code now reads. Re-enable them by publishing the
config they need (get_context().override_server_args seeding, scoped per
test), asserting bag state where the old assertions checked instance
write-through (declare_load_time_override is bag-only), and extending the
per-runner stubs the code genuinely reads (kv_cache_dtype_str,
max_total_tokens, context_len).

The unified-radix-cache file (which grew a large hicache/insert-walk suite
while skipped) is recovered in the same change:

- test_cache_finished_req_strips_thinking (19 parametrized classes) wrote
  strip_thinking_cache onto the ServerArgs instance; the cache reads
  get_serving().strip_thinking_cache — use the serving bag's scoped
  override.
- test_shallower_crossing_backs_up_above_backuped_middle staged its
  broken-backup-continuity setup through insert_host, which now
  deliberately drops refills below an un-backed-up node under
  write-through (host_insert_dropped). Build the same tree state through
  an explicit backup + device eviction.

Every config-namespace-migration deferral is recovered, so the deferral
ratchet (test_migration_deferral_ratchet.py) has done its job and is
retired.

test/registered/unit/model_executor/test_pool_configurator.py

@@ -17,21 +17,6 @@ from sglang.test.ci.ci_register import register_cpu_ci
 register_cpu_ci(est_time=10, suite="base-a-test-cpu")
 
 
-import pytest as _pytest_defer
-
-_DEFER_REASON = (
-    "Temporarily skipped during the ServerArgs config-namespace migration; "
-    "re-enabled once the runtime-config accessor API stabilizes."
-)
-pytestmark = _pytest_defer.mark.skip(reason=_DEFER_REASON)
-
-
-def setUpModule():
-    import unittest
-
-    raise unittest.SkipTest(_DEFER_REASON)
-
-
 @contextlib.contextmanager
 def mock_cpu_env(kv_size=2, tp_size=1, swa_eviction_interval=4):
     """Mock GPU-dependent functions for CPU-only testing.
@@ -116,10 +101,12 @@ def _make_model_runner(
     mc.hf_config = SimpleNamespace(architectures=["LlamaForCausalLM"])
     mc.hf_config.get_text_config = lambda: mc.hf_config
     mc.linear_attn_registry_result = None
+    mc.context_len = 8192
     mr.model_config = mc
     mr.kv_cache_dtype = "fake_bf16"
 
     sa = SimpleNamespace()
+    sa.max_total_tokens = None
     sa.swa_full_tokens_ratio = swa_full_tokens_ratio
     sa.page_size = page_size
     sa.disable_radix_cache = disable_radix_cache