CLI: `fleet.yaml` Configuration Validation

[weyfonk]

As a DevOps Engineer or Developer, I want to validate the syntax and semantic correctness of my fleet.yaml (and other Fleet configuration files) locally via a CLI command before committing changes to version control, so that I can catch configuration errors early, prevent deployment failures, reduce debugging time, and ensure my configurations adhere to best practices and Fleet's schema.

Acceptance Criteria:

• A fleet validate -f <path-to-fleet.yaml> command exists.

  • The command checks for correct YAML syntax.
  • It validates the file against the known schema.
  • It checks for common misconfigurations or deprecated fields.

• The command provides clear, actionable error messages, indicating the problematic file, line number, and a description of the issue.

• A successful validation returns a confirmation message and a zero exit code, making it suitable for integration into pre-commit hooks and CI/CD pipelines.

Related to #1962.

(Original contents from #3771)