Upgrade to Java 25, modernize build/CI, and refactor dynamic compiler internals

[raulgomis]

Motivation

• Move the project to a modern JDK/tooling baseline (Java 25) and modernize CI to run tests on feature branches and master.
• Improve internal robustness by using immutable/final fields, thread-safe collections and clearer error handling.
• Update tests and utilities to use current Java and JUnit APIs for clearer and safer test code.

Description

• CI/workflows: added /.github/workflows/feature-branches.yml and updated /.github/workflows/master.yml to use actions/checkout@v4, actions/setup-java@v4 with JDK 25, and run mvn -B test; added contents: read permissions.
• Build: updated pom.xml to set maven.compiler.release to 25, consolidate to junit-jupiter with property junit.jupiter.version, upgraded maven-compiler-plugin to 3.14.1 and maven-surefire-plugin to 3.5.4.
• Runtime refactors: made many internal fields final, replaced HashMap with ConcurrentHashMap in DynamicClassLoader, removed debug println, simplified and clarified method signatures, and replaced deprecated/fragile calls (e.g. Class#newInstance) with getDeclaredConstructor().newInstance().
• API and utils: improved URI handling to throw IllegalArgumentException for bad URIs, used List.of/List.copyOf for diagnostics, improved getDiagnosticsError formatting, used Files.readString in test utilities, and small naming/signature cleanups (e.g. parameter renames and null-safety checks).
• Tests and test helpers: updated test implementations to JUnit Jupiter idioms (assertThrows, descriptive test names), simplified CompilerExecutor and CompilerTestUtils, and adjusted test inputs/fixtures accordingly.
• Documentation: updated README.md to mention Java 25 and to modernize example usage.

Testing

• Ran mvn -B test (JUnit Jupiter) against the updated project and the unit test suite completed successfully.
• CI workflows added/updated to run mvn -B test on feature branch pushes and PRs to master (configured in the new/updated workflow files).