fix: restore .text attribute when loading text artifacts from GCS

[Maanik23]

Fixes #3157

Summary

When artifacts are saved via Part.from_text() using GcsArtifactService, the .text attribute is None after loading — the content is present in .inline_data instead. This PR fixes the deserialization to correctly restore text artifacts.

Root Cause

GcsArtifactService._save_artifact() stores text parts with content_type="text/plain", but _load_artifact() always used Part.from_bytes() which populates inline_data regardless of content type.

Before (broken):

artifact = types.Part.from_bytes(data=artifact_bytes, mime_type=blob.content_type)
# artifact.text is None, content is in artifact.inline_data

After (fixed):

if blob.content_type and blob.content_type.startswith("text/plain"):
    return types.Part(text=artifact_bytes.decode("utf-8"))
# artifact.text == "original text"

Why startswith instead of ==

Using startswith("text/plain") handles potential charset suffixes like text/plain; charset=utf-8 that cloud storage providers may append.

Consistency

FileArtifactService already handles this correctly (line 473: return types.Part(text=text)). This fix brings GcsArtifactService in line with that behavior.

Test plan

• Added test_text_artifact_roundtrip — parametrized across all three service types (InMemory, GCS, File)
• Asserts loaded.text == original_text after save/load cycle
• Includes descriptive assertion message showing both .text and .inline_data on failure

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[Maanik23]

I have signed the Google CLA at cla.developers.google.com. Could you please recheck?