mcp: regenerate platform_auth signature snapshot for auth_headers(workspace_id=...)

PR-1's auth_headers added an optional workspace_id parameter for
multi-workspace token routing; the signature drift gate
(test_platform_auth_signature_matches_snapshot) caught the change as
expected. Snapshot regenerated to capture the new shape — diff is
visible in the PR for reviewers + template repos that depend on this
surface.

Behavior unchanged: auth_headers() with no arg still routes through
the legacy resolution path (back-compat exact); the workspace_id arg
is opt-in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

workspace/tests/snapshots/platform_auth_signature.json

@@ -4,7 +4,14 @@
       "is_abstract": false,
       "is_async": false,
       "name": "auth_headers",
-      "parameters": [],
+      "parameters": [
+        {
+          "annotation": "str | None",
+          "has_default": true,
+          "kind": "POSITIONAL_OR_KEYWORD",
+          "name": "workspace_id"
+        }
+      ],
       "return_annotation": "dict[str, str]"
     },
     {