Add unit tests for EsbuildCompressor

[bjagg]

Context

PR #303 added EsbuildCompressor.java, a new class that forks npx esbuild to minify JavaScript and CSS streams. The class has no direct unit test coverage.

Why it matters

• The class uses ProcessBuilder and handles timeouts, exit codes, stderr output, and temp file cleanup — all error-prone
• It has a fallback path (currently silent, will change per EsbuildCompressor and minify.js silently fall back to uncompressed output #309) that deserves explicit test coverage
• Regression risk is high if someone refactors the temp file or process handling

Proposed tests

Unit tests under resource-server-core/src/test/java/org/jasig/resource/aggr/:

1. compressJavaScript with a simple JS input — output is smaller than input when esbuild is available
2. compressCss with a simple CSS input — same
3. Fallback path when npx is not on PATH (mock via ProcessBuilder or a test shim)
4. Timeout handling (long-running esbuild call)
5. Non-zero exit code handling (invalid JS input)
6. Temp file cleanup on success and failure paths

Tests should be skipped or marked conditional if Node is not available on the test runner, so CI without Node still passes.

Priority

Low — existing aggregator integration tests provide some coverage indirectly. Filing as a follow-up to PR #303.