add compression-level input for save step

[StephenHodgson]

Description

Adds optional compression-level support and extends tests to validate gzip output size differences between low and high compression settings when saving cache.

Motivation and Context

Allows users to control compression strength (0–9) and now includes a regression test to ensure higher compression produces smaller archives.

How Has This Been Tested?

• npm test -- --runTestsByPath __tests__/actionUtils.test.ts
• npm test -- --runTestsByPath __tests__/actionUtils.test.ts __tests__/saveImpl.test.ts __tests__/saveOnly.test.ts __tests__/save.test.ts

Screenshots (if appropriate):

Types of changes

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (add or update README or docs)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Copilot]

Pull request overview

This PR adds a new compression-level input parameter to the cache and save actions, allowing users to control the compression strength (0-9) when creating cache archives. The feature integrates cleanly with existing code by adding validation functions and setting appropriate environment variables for the underlying compression tools (GZIP and zstd).

Key changes:

• Added getCompressionLevel() and setCompressionLevel() utility functions with validation for values 0-9
• Integrated compression level configuration into the cache save workflow
• Extended test coverage with unit tests and a gzip compression verification test

Reviewed changes

Copilot reviewed 11 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/utils/actionUtils.ts	Added compression level getter/setter functions with range validation
src/saveImpl.ts	Integrated compression level retrieval and application before cache save
src/constants.ts	Added CompressionLevel input constant
action.yml	Added compression-level input to main cache action
save/action.yml	Added compression-level input to save action
save/README.md	Documented the compression-level input parameter
README.md	Added compression-level to inputs documentation and fixed trailing whitespace
tests/actionUtils.test.ts	Added comprehensive tests for compression level functions and gzip behavior
tests/saveImpl.test.ts	Added tests for compression level application and out-of-range handling
tests/save.test.ts	Added test verifying compression level sets environment variables
tests/saveOnly.test.ts	Added test verifying compression level sets environment variables
dist/*	Updated compiled distribution bundles with new functionality

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[Copilot]

Pull request overview

Copilot reviewed 11 out of 15 changed files in this pull request and generated 1 comment.

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[Copilot]

Pull request overview

Copilot reviewed 11 out of 15 changed files in this pull request and generated 1 comment.

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[StephenHodgson]

I decided to pull some of these changes down into the toolkit/cache project instead.
Put back into draft until those changes are released