fix: make PromptVersion.published_at optional

[anxkhn]

px0-python is generated from the OpenAPI spec in px0-ai/px0, so this is ultimately a spec issue: the durable fix belongs in that spec, and this patch corrects the currently published SDK to match. Happy to redo it as a spec-side change if you prefer.

published_at is populated only once a version is set to live. For draft, stable and archived versions the API returns published_at: null, but the field was generated as a required, non-nullable datetime, so from_dict raised ValidationError and the SDK could not read the response.

The underlying fix belongs in the OpenAPI document: mark published_at nullable: true and remove it from required on the PromptVersion schema. This applies the equivalent change to the generated model so it matches what the generator would produce, mirroring APIKey.last_used_at, which already handles the same "Null if ..." case:

-    published_at: datetime = Field(description="Timestamp when status was set to live. Null if draft.")
+    published_at: Optional[datetime] = Field(default=None, description="Timestamp when status was set to live. Null if draft.")

from_dict already reads the value with obj.get("published_at"), and to_dict uses exclude_none=True, so a null or omitted value round-trips cleanly and is dropped on serialization. docs/PromptVersion.md is updated to mark the field optional.

Why it matters

Without this, create_version, promote_version, duplicate_version, get_version, archive_version and list_versions all fail whenever the version they touch is not live (a single non-live element also breaks the whole list_versions array).

Testing

Added tests/test_prompt_version_published_at.py covering a draft version with published_at: null, published_at omitted entirely, a live version keeping its timestamp, to_dict dropping the null field, and a list_versions payload containing a draft element. pytest test tests passes.

Closes #1