CompilationTestHelper: group expectations and reported diagnostics per failing line

[reginaldalfret]

Summary

When a CompilationTestHelper test fails, expectations and diagnostics were reported in disconnected sections: mismatches listed line numbers and expectation strings, followed by All errors: which printed every diagnostic emitted during compilation (including passing lines) in a single dump. When a diagnostic was missing on a line, that line's source code did not appear anywhere in the failure message.

This change:

1. Groups what was expected and what was reported per failing line.
2. For missing diagnostics on line N, prints the expectation followed by /file.java:N: no diagnostic was reported on this line and the source line indented by 4 spaces.
3. For unexpected or text-mismatched diagnostics on line N, prints the expectation followed by the diagnostics reported on line N.
4. Omits diagnostics on passing lines from the failure output.
5. Emits any unattached diagnostics (or diagnostics on unmarked lines without line positions) in an optional closing Other diagnostics: block (omitted when empty).

Fixes #6145.

Testing

• Updated existing CompilationTestHelperTest assertions to reflect grouped failure reporting.
• Added ourKindsOfLineGroupingExpectationAndReportedDiagnostics verifying passing line omission, missing diagnostic source printing, mismatched text reporting, and single reporting of unexpected lines.
• Ran all tests in est_helpers and check_api cleanly via Maven.