[analysis_server] Do not offer CreateClass for method invocations on non-prefix targets

[ayushgupta9906]

Summary

Do not offer the Create class quick fix on method invocations where the target is not an import prefix.

In Dart, an invocation of the form target.name() can only be a class instantiation if target is an import prefix (PrefixElement). Previously, when calling an undefined member on a class (e.g. C.m()), target.staticType evaluated to null, causing CreateClass to erroneously offer to create a top-level class m.

Fixes #64192
Work towards #63444

Changes

• Updated CreateClass.producers in pkg/analysis_server/lib/src/services/correction/dart/create_class.dart to verify that when targetNode == methodName, target must either be null (unqualified invocation) or a PrefixElement. If target != null and not an import prefix, creating a class is rejected.
• Added unit tests in pkg/analysis_server/test/src/services/correction/fix/create_class_test.dart covering lowercase and uppercase static method invocations, as well as preserving support for undefined target invocations (e.g. Foo.bar()).

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Gerrit CL has been approved, please wait for a reviewer to merge it.