fix(eval): raise the Invalid filter error for filters without an equals sign (#826)

## Summary

`parse_filters` looks for the `=` separator with `str.index`, which
raises `ValueError: substring not found` when the separator is missing.
The `-1` check and the `Invalid filter` message that follow it never
run. This change switches to `str.find` so a filter without `=` raises
`ValueError: Invalid filter <filter>`, matching the JavaScript SDK.

## Changes

- Use `str.find("=")` in `parse_filters` so the existing `Invalid
filter` error is reachable.
- Add tests for the error message and for splitting on the first `=`
only.

## Testing

```
cd py
python -m pytest src/braintrust/test_framework.py -k "parse_filters or evaluate_filter"
```

Before the change: 1 failed, 9 passed. After the change: 10 passed.

Author: RaphaelFakhri

py/src/braintrust/framework.py

@@ -1157,7 +1157,7 @@ def deserialize_plain_string_as_json(s: str) -> Any:
 def parse_filters(filters: list[str]) -> list[Filter]:
     result = []
     for f in filters:
-        equals_idx = f.index("=")
+        equals_idx = f.find("=")
         if equals_idx == -1:
             raise ValueError(f"Invalid filter {f}")
         path, value = f[:equals_idx], f[equals_idx + 1 :]

py/src/braintrust/test_framework.py

@@ -1462,6 +1462,15 @@ def test_evaluate_filter_nested_metadata(self):
         f = Filter(path=["metadata", "priority"], pattern=re.compile("^P0$"))
         assert evaluate_filter(datum, f) is True
 
+    def test_parse_filters_splits_on_first_equals(self):
+        filters = parse_filters(["metadata.name=a=b"])
+        assert filters[0].path == ["metadata", "name"]
+        assert filters[0].pattern.pattern == "a=b"
+
+    def test_parse_filters_rejects_filter_without_equals(self):
+        with pytest.raises(ValueError, match="Invalid filter metadata.name"):
+            parse_filters(["metadata.name"])
+
     def test_evaluate_filter_input_field(self):
         datum = EvalCase(input={"text": "hello world"}, metadata={"name": "foo"})
         f = Filter(path=["input", "text"], pattern=re.compile("hello"))