fix(claude-code): honor configured models

[r266-tech]

Summary

• pass the resolved Claude Code model to both SDK option paths
• preserve empty-model behavior by omitting the SDK keyword
• cover standard and tool-enabled paths, including the installed SDK contract

Fixes #2881.

This also makes the existing provider-aware default effective when no per-scope
model is configured; it does not change the default-model table.

Verification

• uv run pytest tests/test_claude_code_llm_isolation.py -q (7 passed)
• uv run pytest tests/test_provider_default_models.py tests/test_per_operation_llm_config.py -q
  (16 passed; one unrelated test could not start embedded Postgres because the
  sparse test environment lacks the optional pg0-embedded package)
• targeted Ruff check and format check
• targeted ty check
• git diff --check
• Codex adversarial review: approved

Thanks @bchumerch for the production reproduction and local validation, and
@ebarkhordar for verifying the current-main call sites and SDK contract.