fix(diff-editor): support originalEditable in inline diff mode (#5302)

[Veeresh Yadav (veeresh4520)]

Description

Fixes #5302.

The original model of an inline diff editor could not be edited even when originalEditable was set to true.

This change fixes the issue while preserving the existing readOnly behavior and adds a regression test to ensure the original model remains editable when the configuration allows it.

Changes

• Fixed editing of the original model in inline diff editors.
• Added a regression test covering originalEditable in inline diff mode.
• Preserved existing behavior for side-by-side diff editors and read-only configurations.

Testing

• Added a regression test for the reported configuration.
• Ran the relevant diff editor tests.
• Ran the applicable repository validation checks.

[Veeresh Yadav (veeresh4520)]

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