sync-diff-inspector: clarify data type support and improve collation guidance

[alastori]

What is changed, added or deleted? (Required)

This PR updates the sync-diff-inspector documentation to fix misleading language about data type support and add empirically-validated guidance to prevent false-positive validation failures.

Key changes:

1. Clarified data type support: Added explicit documentation that JSON, BLOB/VARBINARY, and BIT types are supported, correcting misleading comments suggesting they are not.

2. Improved collation guidance: Enhanced the collation warning with specific examples (MySQL 8.0 utf8mb4_0900_ai_ci vs TiDB utf8mb4_bin) and actionable recommendations to avoid false positives.

3. Documented known false-positive patterns: Added list of scenarios that produce false differences (collation mismatches in VARCHAR/TEXT/JSON, auto-populated TIMESTAMP columns).

4. Added TIMESTAMP handling guidance: Provided concrete approaches for handling auto-populated timestamp columns during validation.

All changes are based on comprehensive empirical validation testing using MySQL 8.0 → TiDB v8.5.4.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR addresses documentation accuracy issues identified through empirical validation
• Related to sync-diff-inspector tool: https://github.com/pingcap/tiflow/tree/master/sync_diff_inspector

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Summary of Changes

Hello @alastori, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly updates the sync-diff-inspector documentation to improve its accuracy and utility. Based on comprehensive empirical validation, the changes clarify which data types are supported and offer detailed guidance on how to avoid common false-positive validation failures, particularly those arising from collation differences and auto-populated timestamp columns. The aim is to provide users with more precise information and practical solutions for robust data comparison between MySQL and TiDB.

Highlights

• Clarified Data Type Support: Explicitly documented support for JSON, BLOB/VARBINARY, and BIT types, correcting previous misleading statements in the documentation.
• Improved Collation Guidance: Enhanced the documentation with specific examples of collation mismatches (e.g., MySQL 8.0 utf8mb4_0900_ai_ci vs TiDB utf8mb4_bin) and actionable recommendations to prevent false-positive validation failures.
• Documented False-Positive Patterns: Added a list of common scenarios that lead to false differences, such as collation mismatches in VARCHAR/TEXT/JSON columns and auto-populated TIMESTAMP columns.
• TIMESTAMP Handling Guidance: Provided concrete approaches for handling auto-populated timestamp columns during data validation to ensure accuracy.

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[gemini-code-assist]

Code Review

This pull request significantly improves the sync-diff-inspector documentation by clarifying data type support and providing detailed guidance on handling collations to prevent false positives. The changes are accurate, well-researched, and add a lot of value for users. I've provided a couple of minor suggestions to enhance readability and ensure consistency with the documentation style guide. Overall, this is an excellent contribution.

[alastori]

PTAL @Frank945946 @joechenrh

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@joechenrh: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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