Update Mist to support Mojo 0.25.7

[thatstoasty]

Checklist

• My recipe.yaml file specifies which version(s) of MAX is compatible with my project (see here for an example). If not, my package is compatible with both 24.5 and 24.6.
• License file is packaged (see here for an example).
• Set the build number to 0 (for new packages, or if the version changed).
• Bumped the build number (if the version is unchanged).

[Copilot]

Pull request overview

This PR updates the Mist package to support Mojo 0.25.7, migrating from MAX dependency specification to explicit mojo-compiler version pinning. The changes align with the repository's convention for version-specific Mojo packages.

Key Changes:

• Version bump from 25.5.0 to 25.7.0
• Introduced mojo_version context variable (=0.25.7) for consistent version management
• Migrated from max dependency to mojo-compiler with explicit version pinning
• Enhanced test configuration with custom script, source files, and explicit requirements

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