Preserve legacy Xarray data_vars settings

[tomvothecoder]

Description

• Closes [Bug]: Preserve legacy Xarray behavior with data_vars="all" #324

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:

• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have added tests that prove my fix is effective or that my feature works
• 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)

[Copilot]

Pull Request Overview

This PR refactors the codebase to centralize legacy Xarray merge settings by introducing a LEGACY_XARRAY_MERGE_SETTINGS constant in the __init__.py module. This replaces repeated inline configuration of compat and join parameters across multiple files with a single source of truth.

• Introduced LEGACY_XARRAY_MERGE_SETTINGS dictionary in e3sm_to_cmip/__init__.py to centralize Xarray legacy behavior settings
• Replaced inline compat="no_conflicts" and join="outer" parameters with the new constant across four files
• Added data_vars="all" to the legacy settings dictionary for consistency

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
e3sm_to_cmip/init.py	Defines LEGACY_XARRAY_MERGE_SETTINGS constant with legacy Xarray merge parameters
e3sm_to_cmip/mpas.py	Replaces inline legacy settings with LEGACY_XARRAY_MERGE_SETTINGS in two locations
e3sm_to_cmip/cmor_handlers/mpas_vars/pso.py	Replaces inline legacy settings with LEGACY_XARRAY_MERGE_SETTINGS
e3sm_to_cmip/cmor_handlers/mpas_vars/pbo.py	Replaces inline legacy settings with LEGACY_XARRAY_MERGE_SETTINGS
e3sm_to_cmip/cmor_handlers/handler.py	Replaces inline legacy settings with LEGACY_XARRAY_MERGE_SETTINGS

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}