fix: harden plugin feature gating (#15020)

1. Use requirement-resolved config.features as the plugin gate.
2. Guard plugin/list, plugin/read, and related flows behind that gate.
3. Skip bad marketplace.json files instead of failing the whole list.
4. Simplify plugin state and caching.

sdk/python/src/codex_app_server/generated/v2_all.py

@@ -3131,6 +3131,7 @@ class ThreadSourceKind(Enum):
     vscode = "vscode"
     exec = "exec"
     app_server = "appServer"
+    custom = "custom"
     sub_agent = "subAgent"
     sub_agent_review = "subAgentReview"
     sub_agent_compact = "subAgentCompact"
@@ -5810,11 +5811,19 @@ class SubAgentSessionSource(BaseModel):
     sub_agent: Annotated[SubAgentSource, Field(alias="subAgent")]
 
 
-class SessionSource(RootModel[SessionSourceValue | SubAgentSessionSource]):
+class CustomSessionSource(BaseModel):
+    model_config = ConfigDict(
+        extra="forbid",
+        populate_by_name=True,
+    )
+    custom: str
+
+
+class SessionSource(RootModel[SessionSourceValue | CustomSessionSource | SubAgentSessionSource]):
     model_config = ConfigDict(
         populate_by_name=True,
     )
-    root: SessionSourceValue | SubAgentSessionSource
+    root: SessionSourceValue | CustomSessionSource | SubAgentSessionSource
 
 
 class Thread(BaseModel):