Add Responses API model constants (ResponsesModels)

[RushitaThakkar]

Summary

Adds recommended model ID constants for the Responses API so callers can use stable names instead of raw strings.

Changes

• src/gradient/responses_models.py — New ResponsesModels class with:
  • GPT_5_2_PRO = "openai-gpt-5.2-pro"
  • GPT_5_1_CODEX_MAX = "openai-gpt-5.1-codex-max"
• src/gradient/__init__.py — Export ResponsesModels in the public API.
• tests/test_responses_models.py — Tests that both constants exist and have the expected string values.

Usageython

from gradient import Gradient, ResponsesModels

client = Gradient()
response = client.responses.create(
model_access_key="...",
input=[{"role": "user", "content": "Hello"}],
model=ResponsesModels.GPT_5_2_PRO,
)