replace hardcoded "summarize" no-selection message with overridable getNoSelectedTextMessage() per plugin

Ann Mary Sruthyrequested to merge
issue/ai-3575346:3575346-ai-ckeditor-hardcoded into 2.0.x

feat: #3575346 AI CKEditor: replace hardcoded "summarize" no-selection message with overridable getNoSelectedTextMessage() per plugin

Closes #3575346

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