Report structured BuilderProblems from compiler diagnostics

[gnodet]

Summary

Maps javax.tools.Diagnostic to BuilderProblem and reports via the new DiagnosticReporter service from Maven 4.1.0, so compiler warnings and errors appear with structured keys, source locations, and severity in the build report and mvnlog --diagnostics output.

Related: apache/maven#12644 — follow-up to apache/maven#12572 (Build Report Foundation).

Changes

• AbstractCompilerMojo: Add @Inject DiagnosticReporter diagnosticReporter
• DiagnosticLogger: Accept DiagnosticReporter in constructor; for each javax.tools.Diagnostic, create a BuilderProblem with:
  • key("compiler:" + diagnostic.getCode()) — e.g. compiler:compiler.warn.unchecked
  • severity mapped from Diagnostic.Kind (ERROR→ERROR, WARNING→WARNING, etc.)
  • source, lineNumber, columnNumber from the diagnostic
• ToolExecutor: Pass diagnosticReporter through to DiagnosticLogger
• pom.xml: Bump mavenVersion to 4.1.0-SNAPSHOT for DiagnosticReporter API

Design decisions

• Per-type dedup key: Each diagnostic uses "compiler:<code>" as the key, so 50 unchecked warnings produce a single summary entry with count=50. Individual per-file details remain in the build log.
• Suppression: Users can suppress compiler diagnostics via -Dmaven.diagnostic.suppress=compiler:* (all) or compiler:compiler.warn.unchecked (specific).
• No behavior change: Existing logging behavior is unchanged — DiagnosticReporter is additive.

Test changes

• Updated test imports for maven-testing package relocation (4.0.0-rc-4 → 4.1.0-SNAPSHOT)
• Added no-op DiagnosticReporter provider in test DI configuration

Test plan

• mvn test — 16 tests pass, 0 failures
• mvn compile — 0 checkstyle violations, spotless clean
• CI — awaiting results

Dependencies

This PR requires Maven core 4.1.0-SNAPSHOT which includes:

• DiagnosticReporter service (apache/maven#12572)
• BuilderProblem.builder() with key(), suggestion(), documentationUrl() fields

[gnodet]

CI status: ⚠️ CI fails as expected — the build requires maven-api-core:4.1.0-SNAPSHOT (for DiagnosticReporter), which is not yet published. The CI matrix tests against 4.0.0-rc-4.

This PR is ready for review but will only pass CI once Maven core #12572 is merged and a SNAPSHOT is deployed.

Tests pass locally against a locally-installed 4.1.0-SNAPSHOT (16/16 pass, 0 checkstyle violations).