fix diff for auto named indexes

[morgo]

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Fixes #645

[Copilot]

Pull request overview

This PR fixes schema diff generation when indexes are declared without an explicit name (e.g., INDEX (col)), by normalizing them to MySQL’s default auto-naming behavior so diffs match what SHOW CREATE TABLE returns.

Changes:

• Auto-assign names to unnamed (non-PRIMARY) indexes during CREATE TABLE parsing using MySQL’s “first column + optional _2/_3…” convention.
• Adjust ALTER TABLE ... ADD INDEX formatting to safely omit index names when empty (as a fallback).
• Add/adjust tests to validate unnamed-index normalization and diff behavior (including duplicates and UNIQUE).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
pkg/statement/parse_create_table.go	Auto-names unnamed indexes during parsing to match MySQL normalization.
pkg/statement/parse_create_table_test.go	Updates expectations for unnamed index names in parsing tests.
pkg/statement/diff.go	Updates ADD INDEX formatting to not emit empty backticked names.
pkg/statement/diff_test.go	Adds coverage for unnamed index normalization and diff stability.

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