[jni] Add JType.isA and JType.as to match ffigen's approach

[imshubham22apr-gif]

Description

This PR adds isA and as methods to JType to match the approach used in package:objective_c (ffigen). Since JNI classes are now extension types, standard Dart is and as checks are not reliable.

Key changes:

• Implemented JType.isA and JType.as for safe runtime type checks and casting.
• Updated JObject.isA and JObject.as to delegate to the new JType methods.
• Removed the previous hack in findClass (as it was related to this type migration).
• Added comprehensive documentation in pkgs/jni/doc/runtime_types.md.
• Updated unit tests to verify the new patterns.

Related Issues

Fixes #3175
Relates to #3174

PR Checklist

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• I've run dart tool/ci.dart --all locally (Note: Environment setup issue locally, relying on GitHub CI).
• All existing and new tests are passing.
• The PR is actually solving the issue.

[liamappelbe]

@imshubham22apr-gif #3093 is a huge refactor to all this infra, so it would be best to wait for that to land before trying to do this.