fix(eval): default RubricContent.text_property to None so rubrics save without 422

[ALDRIN121]

Summary

Fixes a Pydantic-v2 "required Optional" trap that makes saving an eval case fail with HTTP 422 when a rubric's text_property is omitted or empty.

RubricContent.text_property is typed Optional[str] but has no default. Under Pydantic v2, an Optional field without a default is still required — so when the Web UI's GET serializes an eval case with response_model_exclude_none=True (dropping text_property: None) and the browser re-submits it on the save PUT, the field is missing and validation fails with 422 Unprocessable Entity.

This is the exact same bug class as #6336 / the #6515 fix (fix: make InvocationEvent.content optional so the Web UI can save eval cases), which patched InvocationEvent.content but left RubricContent.text_property unaddressed. Related to #7019 (422 persists across 2.6.0–2.8.0 despite #6515).

Root cause

src/google/adk/evaluation/eval_rubrics.py:

class RubricContent(EvalBaseModel):
  text_property: Optional[str] = Field(
      description="..."          # no default -> Pydantic v2 treats as required
  )

Fix

Default text_property to None, matching the accepted InvocationEvent.content fix:

text_property: Optional[str] = Field(
    default=None,
    description="...",
)

Reproduction (before the fix)

rubric = Rubric(rubric_id="r1", rubric_content=RubricContent(text_property=None))
case = EvalCase(eval_id="c1", conversation=[], rubrics=[rubric])
wire = case.model_dump(by_alias=True, exclude_none=True)  # GET
EvalCase.model_validate(wire)                              # PUT -> ValidationError
# pydantic_core.ValidationError: rubricContent.textProperty — Field required

After the fix, the round-trip succeeds and text_property is None.

Testing plan

• pytest tests/unittests/evaluation/test_eval_case.py — 24 passed (2 new tests: test_rubric_content_text_property_defaults_to_none, test_eval_case_with_rubric_missing_text_property_round_trips).
• pytest tests/unittests/evaluation/test_rubric_based_tool_use_quality_v1.py — passed (no regression in rubric consumers).

Note: I ran the focused evaluation tests; the broader tests/unittests/evaluation/ suite requires optional GCS/Vertex deps not installed in the minimal venv.

Signed-off-by: Aldrin Joseph yoaldrinjoseph@gmail.com