fix: preserve Claude Code default budget with real configs

[yunyv]

Summary

Use the documented 1.0 USD default when a Claude Code model's max_budget_usd is omitted or null.

Real LLMModelConfig objects have this attribute set to None by default. The provider's getattr(..., 1.0) fallback therefore does not apply, and the generated command contains --max-budget-usd None. Existing backend tests supplied a numeric budget through a mock config, hiding the problem.

This change handles only None; explicit values and per-call overrides keep their existing behavior. Five regression tests exercise the real config loader and provider factory with the subprocess mocked.

Validation

• Before the fix: provider test file 2 failed, 18 passed. The omitted/null budget regressions failed.
• After the fix: 47 relevant provider/config tests passed.
• Full documented unittest suite: 667 passed, 2 skipped (669 run).
• Black checks for both changed files and git diff --check: passed.
• Independent automated review: provider tests plus four additional offline cases passed; no findings.

The full unit suite ran with test-only guards preventing external network requests and real model CLI subprocesses; its local fake HTTP server was allowed. No live Claude Code invocation or paid model call was tested. The model-backed integration suite was not run.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}