fix: use empty-text sentinel in ArtifactStreamer.finalize for spec compliance

Revert unrelated changes to .pre-commit-config.yaml, pyproject.toml,
and uv.lock. Use Part(text='') instead of empty parts list in finalize()
to satisfy the spec's "at least one part" requirement.

Author: kevinlu310

.pre-commit-config.yaml

@@ -31,7 +31,6 @@ repos:
   # ===============================================
   # Python Hooks
   # ===============================================
-
   # Ruff for linting and formatting
   - repo: https://github.com/astral-sh/ruff-pre-commit
     rev: v0.12.0

pyproject.toml

@@ -117,7 +117,6 @@ dev = [
   "types-protobuf",
   "types-requests",
   "pre-commit",
-
   "trio",
   "uvicorn>=0.35.0",
   "pytest-timeout>=2.4.0",

src/a2a/utils/artifact.py

@@ -163,6 +163,9 @@ def finalize(self) -> TaskArtifactUpdateEvent:
             artifact=Artifact(
                 artifact_id=self._artifact_id,
                 name=self._name,
-                parts=[],
+                # Spec requires >= 1 part; use empty-text sentinel since
+                # finalize carries no new content.
+                # https://github.com/a2aproject/A2A/issues/1231
+                parts=[Part(text='')],
             ),
         )