feat(prompts): expose create_time and add creator attribution to prom…

[priyankaiitg]

…pt versions

Changes Summary

• Added create_time (datetime) and creator (str) fields to PromptVersionMetadata dataclass.
• Added optional creator parameter to create_version() so callers can record who saved a prompt version (e.g. an email address).
• Updated list_versions() to populate create_time from DatasetVersion.create_time (already returned by the API but previously discarded) and decode creator from the version display name.
• Added _encode_display_name / _decode_display_name helpers and _CREATOR_DELIMITER constant to implement the encoding convention.
• Threaded the creator argument through _create_prompt_resource, _create_prompt_version_resource, and _create_dataset_version.
• Added import datetime to support timezone-aware datetime conversion.

Limitation

The Vertex AI DatasetVersion API has no native creator/user field in its proto. The metadata field is output-only (auto-populated by the backend), leaving display_name as the only user-writable field on a version. Creator identity is therefore encoded as a suffix in display_name using the delimiter __creator__ and transparently decoded on read.

Versions saved via the Vertex AI Console or older SDK versions will have creator=None. Only versions created through this SDK with an explicit creator argument will carry attribution.

Recommendation

File a feature request with the Vertex AI team to add a first-class creator / last_modified_by field to the DatasetVersion proto. That is the proper fix for full, reliable, historical user attribution across all clients (SDK, Console, API). Once available, the SDK encoding workaround here can be removed and the field populated directly.

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Fixes #<issue_number_goes_here> 🦕

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[priyankaiitg]

#6414