feat(cz-git,plugin-loader): add apiExtraBody option

[yohkuri]

Related ISSUE

#261

Type Of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📝 Document (This change requires a documentation update)
• 🎨 Theme style (Theme style beautification)
• ⚠ Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🔨 Workflow (Workflow changes)

Clear Describe

• feat: add apiExtraBody option to adjust the AI request body
• docs: add apiExtraBody option document (en / zh)

Description

useModelStrategy() builds one fixed request body for every model, and none of its fields can be
reached from config. As reported in #261, models which require max_completion_tokens therefore
cannot be used at all: they reject max_tokens at any value, and accept only the default
temperature.

apiExtraBody is merged into the request body after the defaults are built.

The one design point worth reviewing: a null value removes the field instead of overriding it.
A plain merge is not enough here — sending max_tokens alongside max_completion_tokens still
fails, and max_tokens: null fails as well, so the key has to be absent from the JSON entirely.
temperature, by contrast, could be repaired by overriding it with 1; null just keeps the two
cases uniform.

// ~/.config/.czrc
{
  "apiModel": "gpt-5.6-luna",
  "apiExtraBody": {
    "max_tokens": null,
    "temperature": null,
    "max_completion_tokens": 4096
  }
}

Notes on the implementation:

• In aiLoader() the key is spread only when it is set, so an unset value in ~/.config/.czrc
  cannot clobber a project-level apiExtraBody.
• useModelStrategy is now exported so it can be unit tested. It is not re-exported from
  generator/index.ts, so the public API is unchanged.
• No JSON schema change: the AI options (apiModel, apiEndpoint, …) are not part of
  scripts/czrc-schema.d.ts today, so apiExtraBody follows the same treatment. Happy to add them
  all if you would prefer.

⚠ Please review the Chinese documentation

The Chinese docs in this PR — docs/zh/config/engineer.md and docs/zh/recipes/openai.md — were
written with AI assistance. I do not read Chinese and cannot verify the wording myself, so
please review those two files closely rather than trusting them. I would much rather you correct
or rewrite them than merge phrasing I am unable to check.

If you prefer, I can drop the zh changes from this PR entirely and leave them to you.

Test Case

Unit tests — added packages/cz-git/__tests__/api.test.ts (6 cases): default payload unchanged,
add a field, override a field, remove a field via null, the max_completion_tokens scenario, and
that the option object is not mutated.

pnpm test:run
# Test Files  13 passed (13)
#      Tests  116 passed (116)

pnpm lint

Against the real API — loaded the config above from ~/.config/.czrc through configLoader()
→ generateOptions() → useModelStrategy(), then posted the resulting body to
api.openai.com/v1/chat/completions:

config	resulting payload keys	HTTP
apiModel: gpt-5.6-luna + the apiExtraBody above	model, messages, stream, top_p, n, max_completion_tokens	200
apiModel: gpt-4o-mini, no apiExtraBody	model, messages, stream, top_p, temperature, max_tokens, n	200

The second row is the regression check: with no apiExtraBody set, the request body is identical to
what it was before this change.

---

Unrelated note, in case it is useful: 4 tests in
packages/@cz-git/plugin-loader/__tests__/loader.test.ts fail on my machine both with and without
this change, because aiLoader() reads the real $HOME/.config/.czrc. They pass with a clean
HOME, and CI on main is green — so it is not a regression here, but it may be worth isolating
that test from the developer's home config.

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[Zhengqbbb]

LGTM 👍