fix: convert backed enum tool arguments

[mikemikimike]

Summary

• Convert string/integer values returned for tool arguments into backed enum instances expected by the callable.
• Support both positional and named arguments.
• Surface invalid enum values as parameter validation errors.
• Add regression coverage for positional, named, and invalid values.

Closes #1016

Validation

• git diff --check passed.
• Local Pest/PHPStan could not be run because this environment has no PHP/Composer installation and no vendor directory.
• GitHub Actions should run the repository's full Pest matrix and PHPStan checks.

[mikemikimike]

Validation update:

• git diff --check: passed.
• Local Pest/PHPStan: not runnable in the current environment because PHP, Composer, and vendor/ are unavailable.
• Full repository CI: the Formatting, PHPStan, and Tests workflow runs were created, but all are currently action_required because this fork PR requires maintainer approval before workflows execute. No full-context test result is being claimed yet.

The implementation covers positional and named backed-enum arguments, and converts invalid enum values into the existing parameter-validation error path.

[mikemikimike]

Final local validation completed after installing PHP 8.3.33 and Composer 2.10.2 in a temporary project tool directory:

• Full Pest suite (single-process, memory_limit=-1): 1465 passed, 8 skipped, 4832 assertions.
• PHPStan: passed, no errors.
• Pint on changed files (src/Tool.php, tests/ToolTest.php): passed.
• git diff --check: passed.

The initial parallel Pest run hit the Windows worker's default 128MB PHP memory limit in the existing ArchTest worker; rerunning the same full suite single-process completed successfully. The PR branch remains clean at commit 70a26a7f.