[MNG-8708] Fix Maven 4 parent inference

[goutamadwant]

Fixes #10377.

This fixes Maven 4.1.0 parent inference for reactor projects when parent coordinates are partially omitted.

The file-to-raw model transformation now:

• infers a missing parent version from the matching reactor model before raw-model validation;
• preserves the inferred version's source location.

Parent validation now checks whether groupId or artifactId was explicitly declared before warning about combining coordinates with relativePath. Values inferred from a relative parent POM no longer trigger the warning.

The existing warning remains in place when a POM explicitly declares both a non-empty relativePath and parent coordinates.

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Tests

• mvn verify

• MavenITmng8708ParentInferenceTest: 2 tests passed

• MavenITmng8708ParentInferenceTest,MavenITmng8294ParentChecksTest: 8 tests passed

• maven-impl: 550 tests passed, 4 skipped

• Reporter reproduction: all three reactor projects build, and only the explicitly combined parent declaration emits the warning

• mvn -Prun-its verify: source modules passed; the broad Core IT suite reported two unrelated DI fixture errors because org.apache.maven:maven-testing:4.1.0-SNAPSHOT was unavailable

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[gnodet]

Well-structured fix for MNG-8708. The inferParentVersion method correctly follows the same structural pattern as the existing inferDependencyVersion (reactor lookup, version fallback to parent's parent, location preservation), and the switch from value-based to location-based checking in the validator correctly distinguishes between user-declared and resolution-inferred coordinates.

The PR handles all three MNG-8708 scenarios: (1) path-only children no longer get false warnings, (2) explicit coordinates + relativePath still produce the warning, and (3) children specifying only groupId/artifactId can now infer the parent version from the reactor.

A few non-blocking observations:

• Mixin validation consistency (line ~378 in DefaultModelValidator): The analogous mixin validation still uses the old value-based pattern (mixin.getGroupId() != null && !mixin.getGroupId().isEmpty()). If mixin resolution follows the same inference path, it may need the same location-based treatment in a follow-up.

• Null safety in inferParentVersion (line ~660 in DefaultModelBuilder): If versionLocation is null (reactor parent has no explicit <version> and no parent), the Builder.location() call silently does nothing. Safe in practice since a valid parent must have a version, but a defensive comment would improve clarity.

• Test coverage: The test covers the two main MNG-8708 scenarios but doesn't exercise the fallback path where the parent model's version is inherited from its own parent (lines 662-665). A three-level test (grandparent with version → versionless parent → child inferring) would strengthen coverage.

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Claude Code on behalf of gnodet