fix(server): show falsey defaults in CLI help

[cupkk]

add_args_from_model only appends a default value when the value is truthy. As a result, CLI help omits valid defaults such as False for --flash_attn and 0 for several numeric settings.

Check for None instead, while still excluding required fields. The regression test covers boolean false, numeric zero, a truthy default, and a required field.

Validation:

• isolated Pydantic/argparse regression check
• python -m compileall -q llama_cpp/server/cli.py tests/test_server_cli.py
• git diff --check

The full suite was not run because this minimal environment does not contain the compiled llama.cpp extension or the project test dependencies.

[Copilot]

Pull request overview

This PR fixes CLI help generation for the server by ensuring Pydantic field defaults that are “falsy” (e.g., False, 0) are still displayed in argparse help text, rather than being omitted due to truthiness checks.

Changes:

• Update add_args_from_model to append defaults when field.default is not None (while still excluding required fields).
• Add a regression test covering boolean False, numeric 0, a truthy default, and a required field.
• Add an Unreleased changelog entry for the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
llama_cpp/server/cli.py	Fixes help-text default rendering by checking default is not None instead of truthiness.
tests/test_server_cli.py	Adds coverage to ensure falsy defaults appear in generated CLI help.
CHANGELOG.md	Documents the fix under Unreleased.

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