Add Responses API example and documentation

[RushitaThakkar]

Summary

Adds a runnable example and documentation for the Responses API: tool-calling example script, updated create() docstrings, and a short README subsection.

Changes

Example

• examples/responses_tool_calling.py — Script that:
  • Uses client.responses.create() with a function tool (get_weather).
  • Sends a user message, handles a function_call in the output, appends function_call_output, calls create again, and prints the final output_text.
  • Uses ResponsesModels.GPT_5_1_CODEX_MAX.
  • Reads GRADIENT_MODEL_ACCESS_KEY from the environment (optional python-dotenv for a .env file).

Docstrings

• src/gradient/resources/responses/responses.py — Sync and async create() docstrings now document the model argument and point to ResponsesModels (e.g. ResponsesModels.GPT_5_2_PRO) for recommended model IDs.

README

• README.md — New bullet under “The Gradient SDK provides clients for”: Responses API — client.responses with tools (e.g. GPT 5.2 Pro, 5.1 Codex Max), with a link to examples/responses_tool_calling.py.

Testing

• No new unit tests (doc/example only). Example can be run manually with GRADIENT_MODEL_ACCESS_KEY set:
  python examples/responses_tool_calling.py