fix(mcp): guard DbTokenStorage against non-dict oauth_tokens JSON (#5107)

_load() returned whatever json.loads() produced without checking it was a
dict; _update() did the same before assigning data[key] = value. If the
oauth_tokens column ever held a JSON array or primitive (DB corruption,
manual edit, migration drift), _load()'s callers crashed with
AttributeError on .get(), and _update() crashed with TypeError trying to
item-assign into a list/string/int.

Validate the parsed value is a dict in both methods, falling back to {}
otherwise - same recovery behavior already used elsewhere in the codebase
for this exact JSON-blob-is-not-a-dict shape (_parse_tool_args,
_is_sensitive_path's siblings).

Adds 3 regression tests for _load, get_tokens, and _update against a
non-dict oauth_tokens value.

Fixes #5082

src/mcp_oauth.py

@@ -96,7 +96,9 @@ class DbTokenStorage:
         try:
             srv = db.query(McpServer).filter(McpServer.id == self.server_id).first()
             if srv and srv.oauth_tokens:
-                return json.loads(srv.oauth_tokens)
+                parsed = json.loads(srv.oauth_tokens)
+                if isinstance(parsed, dict):
+                    return parsed
         finally:
             db.close()
         return {}
@@ -111,6 +113,8 @@ class DbTokenStorage:
             if srv is None:
                 return
             data = json.loads(srv.oauth_tokens) if srv.oauth_tokens else {}
+            if not isinstance(data, dict):
+                data = {}
             data[key] = value
             srv.oauth_tokens = json.dumps(data)
             db.commit()