Add direct tests for setup action orchestration

[brunoborges]

Summary

Add direct unit tests for the main orchestration in src/setup-java.ts. The distributor implementations have extensive coverage, but the action entrypoint itself currently runs on import and is not directly exercised by the test suite.

Proposed changes

• Export run() and invoke it only when the module is the action entrypoint, following the pattern used by src/cleanup-java.ts.
• Mock action inputs and collaborators to test orchestration without network access or JDK downloads.
• Cover required-input validation for java-version, java-version-file, and distribution.
• Cover version-file distribution inference.
• Cover single- and multi-JDK installation order, Maven toolchain IDs, and final action outputs.
• Cover sequencing of problem matcher, authentication, Maven arguments, and cache restoration.
• Cover failure propagation through core.setFailed.

Acceptance criteria

• Importing src/setup-java.ts in a test does not execute the action.
• The action entrypoint has focused tests for the success and failure paths above.
• Existing distributor and end-to-end behavior remains unchanged.
• Coverage protects input parsing and collaborator sequencing from regressions.