Support compatible Kotlin AST diff trees

[thromel]

Fixes #1100.

Why

Issue #1100 shows a real mismatch in AST-diff for Java-to-Kotlin changes. Java files use the JDT-style GumTree labels, while Kotlin files were still using tree-sitter labels such as class_declaration, function_declaration, and property_declaration. That made the two trees harder to match even when the source-level change was a straightforward Java-to-Kotlin conversion.

This PR changes AST-diff mode only. Refactoring detection still uses the existing Kotlin PSI/UML pipeline.

What changed

• Added a Kotlin PSI-based AST-diff visitor that emits Java-compatible GumTree nodes, including CompilationUnit, PackageDeclaration, ImportDeclaration, TypeDeclaration, FieldDeclaration, MethodDeclaration, SingleVariableDeclaration, Block, ReturnStatement, MethodInvocation, InfixExpression, and comments.
• Switched Kotlin AST-diff generation to that visitor when astDiff=true.
• Kept Java-to-Kotlin detection based on language identity instead of declaration node labels. Kotlin now intentionally emits TypeDeclaration, so comparing declaration labels would hide Java-to-Kotlin diffs.
• Hardened matcher paths that can see missing trees after pruning or location lookup.
• Made decorator checks use the destination language constants.
• Kept Nikos's recent operator work compatible with the new Kotlin AST shape: Kotlin infix operators and assignment operators now use the same compatible operator node names as Java, and initialized Kotlin properties expose their = as ASSIGNMENT_OPERATOR.
• Regenerated the Java-to-Kotlin AST-diff fixtures for the OkHttp and IntelliJ commits covered by the test data.
• Fixed wildcard Kotlin import ranges so a QualifiedName labeled kotlin.math.* covers the full source text, including .*.

Why the fixture diff is large

No fixture files were removed. The large deletion count is inside generated JSON snapshots.

The old Kotlin AST-diff tree came from tree-sitter and included many low-level syntax nodes. The new tree is smaller and Java-compatible. For example, Kotlin now emits CompilationUnit, TypeDeclaration, MethodDeclaration, FieldDeclaration, VariableDeclarationFragment, INFIX_EXPRESSION_OPERATOR, and ASSIGNMENT_OPERATOR instead of tree-sitter-specific nodes such as source_file, class_declaration, function_declaration, and property_declaration.

That means the expected mapping snapshots had to be regenerated. The changed fixture folders are limited to the two commits listed in java2kotlin.json.

Behavior notes

The new visitor is compatibility-oriented. It is not trying to model every Kotlin PSI node as a perfect JDT equivalent. It maps the Kotlin constructs that matter for the current AST-diff path to Java-style labels, then falls back to generic expression or statement wrappers when a construct does not have a dedicated mapping yet.

That keeps the issue fix scoped: Java-to-Kotlin AST diffs now get comparable tree roots and common declaration/body structure, without changing the refactoring model or the non-AST-diff Kotlin path.

Review feedback addressed

Claude Code with the Opus model alias reviewed the change before the PR was opened. I addressed the concrete findings around in-body comments, non-ASCII source offsets, and avoiding silent visitor failures.

puku-cli with the Opus model reviewed the PR after it was opened. One finding held up against the code: wildcard imports had a label/source-span mismatch. This PR now ranges the full wildcard import text and bounds contained-node lookup by the owning PSI element.

Nikos's follow-up commit on master added Java-to-Kotlin operator mappings for infix operators, assignment operators, and variable declaration =. This branch is rebased on that work and adapts it to the compatible Kotlin AST: those operators are now represented with compatible node names instead of tree-sitter operator node names.

The puku review also flagged cross-language detection as a possible regression. I checked that against the previous behavior. Before this PR, Java and Kotlin were already treated as cross-language because Java used TypeDeclaration and Kotlin used tree-sitter declaration labels. The new language-identity check preserves that behavior after Kotlin starts emitting Java-compatible declaration labels.

Tests

• git diff --check
• ./gradlew test --tests org.refactoringminer.astDiff.tests.KotlinCompatibleTreeVisitorTest --tests org.refactoringminer.astDiff.tests.JavaToKotlinDiffTest --no-daemon
• ./gradlew test --tests org.refactoringminer.astDiff.tests.TreeFromParserTest --tests org.refactoringminer.astDiff.tests.TreeMatcherTest --tests org.refactoringminer.astDiff.tests.KotlinCompatibleTreeVisitorTest --tests org.refactoringminer.astDiff.tests.JavaToKotlinDiffTest --tests org.refactoringminer.astDiff.tests.PythonDiffTest --tests org.refactoringminer.astDiff.tests.TypeScriptDiffTest --no-daemon
• ./gradlew test --tests org.refactoringminer.test.TestKotlinDatasetRefactorings --no-daemon