Add explicit --cargo-test-arg option for test arguments

[Copilot]

Adds an explicit --cargo-test-arg command-line option for passing arguments to cargo test, complementing the existing -- args format.

Background

Previously, the only way to pass arguments to cargo test was using the trailing arguments format:

cargo mutants -- --all-targets --no-fail-fast

While this works, it's not as explicit and discoverable as a named option.

Changes

This PR adds a new --cargo-test-arg option that can be repeated:

cargo mutants --cargo-test-arg=--all-targets --cargo-test-arg=--no-fail-fast

Both formats continue to work and can even be combined:

cargo mutants --cargo-test-arg=--all-targets -- --nocapture

When multiple sources provide test arguments (new option, --, and config file), they are combined in the following order:

1. Arguments from --cargo-test-arg
2. Arguments after --
3. Arguments from additional_cargo_test_args in .cargo/mutants.toml

Testing

• Added 6 unit tests covering all combinations of the three argument sources
• Added 3 integration tests verifying command-line behavior
• All existing tests continue to pass
• Verified backward compatibility with the -- args format

Documentation

Updated book/src/cargo-args.md with examples of the new option and explanation of how arguments are combined.

Fixes #505

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Explicit option for test args</issue_title>
<issue_description>Split from #505:

Today you can pass arguments to the test runner with cargo mutants -- --extra-test-args.

It might be nice to also have an explicit option for this, e.g. cargo mutants --cargo-test-arg=--max-fail=2.

    /// Pass remaining arguments to cargo test after all options and after `--`.
    #[arg(last = true, help_heading = "Execution")]
    cargo_test_args: Vec<String>,
```</issue_description>

<agent_instructions>
Add tests for this in options.md.

This should go into both the config and command-line options. Test that the current format is still supported.
</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #536

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