[Chore]: Standardize Java 25 setup and documentation #378

What

Update project documentation and workspace tooling to standardize on Java 25 for all contributors. This includes: updating copilot-instructions.md and README.md to specify Java 25; providing instructions and examples for pinning a JVM identifier with SDKMAN; and adding a devcontainer.json that installs Java 25 automatically with a fallback SDKMAN install.

Why

Ensures consistent environment setup, reduces onboarding friction, improves reproducibility, and makes workspace setup easier for Codespaces, devcontainers, and local development.

Notes

Use SDKMAN to pin JVM installs (e.g. sdk install java 25.0.1-tem) and advise contributors to run sdk list java for platform-specific IDs. The devcontainer.json should include both the official Java feature and SDKMAN, with a postCreateCommand fallback. Update documentation to reflect Java 25 as the minimum required version.