refactor: dedup index code

[tianzhou]

No description provided.

[Copilot]

Pull Request Overview

This PR refactors duplicated index handling code by extracting shared logic into a common function. The refactoring consolidates identical index modification patterns used in both table and materialized view processing.

Key changes:

• Created a shared generateIndexModifications function to handle index drops, adds, and online replacements
• Added a helper generateIndexComment function for comment operations
• Replaced duplicated code in both table and view diff generation with calls to the new shared function

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/diff/view.go	Replaced inline index modification logic with call to shared function
internal/diff/table.go	Replaced duplicated index handling code with call to shared function
internal/diff/index.go	Added new shared functions for index modifications and comment generation

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