Phase 4c: add Java local runner

[SPerekrestova]

Summary

Adds java to IMPLEMENTED_LANGUAGES and executes Java submissions through Java source-file mode:

java: {
  extension: "java",
  sourceFileName: "Solution.java",
  probe: { cmd: "java", args: ["-version"] },
  buildArgs: (sourcePath) => ({ cmd: "java", args: [sourcePath] })
}

The fixed Solution.java source name supports standard LeetCode-style public class Solution snippets while keeping the same isolated temp HOME, timeout, clean-env, sandbox wrapping, and stdout/stderr capture path used by the Python/Go runners.

Updates runner tool copy and runtime-gated unit/e2e coverage so Java executes when a JDK/JRE is present and cleanly reports LANGUAGE_RUNTIME_NOT_FOUND otherwise. Verified locally with npm run build, npm run test:types, npx prettier --check ., focused runner/integration/e2e tests, npm test, npm run test:e2e, and npm run test:all.

Link to Devin session: https://app.devin.ai/sessions/3e6f3e63f6e54852b9bf6da1c792774f
Requested by: @SPerekrestova

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[SPerekrestova]

LGTM

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