fix: Clear error message for non-existent prompts instead of HTTP 404

[sahusiddharth]

Resolves: #6746

• Catch HTTP 404 responses in the prompt retrieval method
• Raise a descriptive ValueError with a clear message indicating the prompt wasn't found
• Maintain all other HTTP error behavior unchanged

Minimal code to reproduce

from phoenix.client import Client

px_client = Client()

prompt_name = "non existant prompt"
px_client.prompts.get(prompt_identifier=prompt_name)

---

Note

Map 404s from prompt retrieval to a clear ValueError in both sync and async clients while preserving other HTTP errors.

• Client prompts API:
  • Prompts.get and AsyncPrompts.get: catch httpx.HTTPStatusError 404 and raise ValueError("Prompt not found: ..."); other errors unchanged.
  • Docstrings updated to document ValueError in Raises.

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[cursor]

Bug: Error Message Mismatch in URL Lookup

The ValueError message for 404 responses in the get methods uses prompt_identifier or prompt_version_id. This expression's priority conflicts with the URL construction logic, which prioritizes prompt_version_id when both are provided. This results in misleading error messages, referencing the identifier that wasn't actually used for the lookup.

Additional Locations (1)

• packages/phoenix-client/src/phoenix/client/resources/prompts/__init__.py#L391-L392