Add dataset validation utilities

[tomvothecoder]

Description

Adds xcdat.validate_dataset() as a non-mutating preflight check for common CF axis and coordinate-bounds problems. This gives users structured, actionable diagnostics before downstream xCDAT operations fail.

Closes #842

• Add structured validation issues, results, and aggregate error reporting
• Detect missing, conflicting, and ambiguous CF axis metadata
• Validate bounds references, dimensions, and vertex counts
• Export and document the public API, with focused regression coverage

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (23bc25d) to head (f93ddc4).

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           18        19     +1     
  Lines         1993      2116   +123     
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+ Hits          1993      2116   +123     

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[Copilot]

Pull request overview

This PR introduces a new public validation API (xcdat.validate_dataset) that performs non-mutating, CF-aware preflight checks on datasets. It returns structured diagnostics (issues, warnings/errors) so users can identify CF axis/metadata and bounds problems before running downstream xCDAT operations.

Changes:

• Added xcdat.validation module with validate_dataset(), structured issue/result types, and an aggregated DatasetValidationError.
• Implemented validation rules for CF axis mapping conflicts/ambiguity and coordinate bounds presence/shape/vertex checks.
• Exported the new API from the top level and documented it in the API reference + FAQ, with focused pytest coverage.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
xcdat/validation.py	New validation implementation and public diagnostics types/errors.
xcdat/__init__.py	Re-exports validation API types and function at top-level xcdat.*.
tests/test_validation.py	Adds regression/unit tests for validation results, axis checks, bounds checks, and exports.
docs/getting-started-guide/faqs.rst	Documents how to run dataset validation and interpret results.
docs/api.rst	Adds validate_dataset to top-level API and documents validation-related public types.