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:<code>:<source>:<line>") — per-location dedup key, e.g. compiler:compiler.warn.unchecked:src/main/java/Foo.java:42
  • 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-location dedup key: Each diagnostic uses "compiler:<code>:<source>:<line>" as the key, so each unique file+line gets its own entry in the build report. This preserves per-file detail — 10 unchecked warnings across 10 files produce 10 separate entries, not one collapsed entry with count=10.
• Suppression: Users can suppress compiler diagnostics via -Dmaven.diagnostic.suppress=compiler:* (all) or compiler:compiler.warn.unchecked (specific prefix).
• 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
• End-to-end verified: built Maven 4.1.0-SNAPSHOT distribution + this plugin against a test project producing 12 compiler warnings across 2 files → 10 per-location structured entries in build report JSON
• CI — expected to fail until Maven 4.1.0-SNAPSHOT is published (needs DiagnosticReporter API from apache/maven#12572)

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).