[ZEPPELIN-6735] Fix code editor model leaks and diff updates

[miinhho]

What is this PR for?

The shared code editor leaves its Monaco text models behind when destroyed. This affects the regular paragraph editor.

In diff mode, the initial original and modified text are reversed. Value changes create new models, and changing only nzOriginalText does not update the original text.

This PR disposes of component-owned models, fixes the diff orientation, and updates the existing models when either input changes.

What type of PR is it?

Bug Fix

Todos

• Fix model disposal and diff initialization and updates.
• Add regression tests.

What is the Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-6735

How should this be tested?

Run the regression tests from zeppelin-web-angular:

npm run test:shell -- src/app/share/code-editor/code-editor.component.spec.ts

Make sure that the tests pass for these behaviors:

• Diff mode displays the original text on the left and the modified text on the right.
• Value changes update the existing models rather than create new ones.
• Changing only nzOriginalText leaves the modified text unchanged.
• Destroying the component disposes of its models but leaves caller-owned models intact.
• Normal-mode writes update the attached model after a caller replaces it.

Questions:

• Does the license files need to update? No.
• Is there breaking changes for older versions? No.
• Does this needs documentation? No.

[voidmatcha]

LGTM 👍