feat: add xarray dataset coordinate endpoint

[hrodmn]

Technical Context

• Related Issues: resolves Expose xarray coordinate values #1503
• Breaking Changes: no

Description

Adds /dataset/coordinates/{name} endpoint to retrieve coordinate values for an xarray dataset dimension

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Checklist

• Linting: Code is formatted and linted (uv run pre-commit run --all-files)
• Tests: Tests pass. I have included new tests for these changes where applicable.
• Edge Cases: I have manually verified "unhappy paths" and edge cases beyond the basic success criteria (e.g., database connection timeouts, malformed input, strict mapping rejections).
• Documentation: I have updated README.md to reflect any new environment variables, configuration changes, or breaking updates.
• Accountability: I can explain the implementation logic for every line of code submitted.

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AI tool usage

• AI (Copilot or something similar) supported my development of this PR. Use of AI tools HAVE TO be indicated.

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[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'TiTiler performance Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.30.

Benchmark suite	Current: f8f0244	Previous: 9669764	Ratio
WebMercator longest_transaction	0.2 s	0.14 s	1.43

This comment was automatically generated by workflow using github-action-benchmark.

[vincentsarago]

thanks @hrodmn

might makes sense to also add /coordinates to return the list of available coordinates 🤷

[hrodmn]

might makes sense to also add /coordinates to return the list of available coordinates 🤷

I added /dataset/dimensions that returns {"dimension_name": dimension_size", ...}

[vincentsarago]

🚀