[chore] Add no-getattr rule; refine no-dataclasses rule (#29871)

.claude/rules/no-getattr-defensive.md

@@ -0,0 +1,39 @@
+---
+paths:
+  - "**/*.py"
+---
+
+# Don't use `getattr` / `hasattr` for defensive access
+
+Over-defensive `getattr(obj, "field", default)` / `hasattr(obj, "field")` hide
+errors and defeat strict type checking. If a field is always present, accessing
+it defensively is confusing and masks real bugs. Prefer:
+
+1. **`isinstance` for type narrowing** — check the type, then access fields directly:
+
+   ```python
+   if (
+       isinstance(obj, (TokenizedGenerateReqInput, TokenizedEmbeddingReqInput))
+       and obj.mm_inputs
+   ):
+   ```
+   (see `python/sglang/srt/managers/mm_utils.py`)
+
+2. **Always set the field (to `None` if needed), then do a `None` check** — the
+   field should always exist, so a `None` / non-`None` check is enough:
+
+   ```python
+   obj.field = None   # in __init__ / construction
+   ...
+   if obj.field is not None:
+       ...
+   ```
+
+Bad — `server_args` always has `revision`, so `getattr` is misleading and swallows
+a real `AttributeError` if the field is ever renamed:
+
+```python
+revision=getattr(server_args, "revision", None),   # BAD
+revision=server_args.revision,                     # GOOD
+```
+(see `python/sglang/srt/managers/template_detection.py`)