Fix default model options when constructing Prompt directly

[oodadoudou]

Directly constructing llm.Prompt("hello", model) leaves prompt.options as a dict. Providers that access option attributes then raise AttributeError; this also reproduces with the built-in OpenAI Responses provider on current main.

Use the model's Options() for the existing empty/default fallback. Explicit Options instances remain unchanged. Four regression cases exercise the public Prompt/Response path using the existing mocked HTTP response fixture: omitted options, None, an empty dict, and a populated Options instance.

Fixes #1028.

Validation (Python 3.12.9):

• Before the fix, all three default-option cases fail on prompt.options.json_object; the explicit-options control passes. After the fix, all 13 options tests pass.
• Full suite: 1,128 passed (18 warnings).
• Black, Ruff, mypy and package build pass.
• Local tests use httpx2-pytest==1.0.1: version 2.0.0 renames the httpx_mock fixture required by the existing suite. No dependency files changed.
• Cog reports an existing version mismatch in unchanged docs/fragments.md (0.33) versus pyproject.toml (0.34), also present on the base commit 2f49eec9.

Implemented and tested with Codex assistance.