Preserve partial index predicates in migrations

[ebrake]

Summary

• Preserve Index(where=...) predicates in migration schema snapshots.
• Carry partial-index predicates through Rust migration metadata and generated migration SQL.
• Emit partial-index SQL for PostgreSQL and SQLite.
• Reject MySQL partial indexes with a clear error instead of silently dropping predicates.
• Detect changed index definitions by rebuilding the index.

Tests Added

• Added a focused schema extraction regression test; verifies Index(where=...) survives extract_current_schema()

  • test_extract_partial_index_where_in_snapshot

• Added Python migration SQL coverage; verifies PostgreSQL/SQLite render WHERE ... and MySQL raises a clear unsupported-dialect error.

  • test_create_partial_index
  • test_create_partial_index_mysql_rejected

• Added full Python migration pipeline coverage; verifies model -> snapshot -> diff -> generated migration -> SQL keeps the partial-index predicate.

  • test_create_partial_index_preserves_where

• Added Rust migration diff and SQL coverage; verifies predicate changes rebuild indexes, whitespace-only differences are ignored, partial-index SQL is emitted for supported dialects, and JSON round-tripping preserves normalized predicates.

  • test_compute_diff_detects_partial_index_predicate_change
  • test_compute_diff_ignores_partial_index_predicate_whitespace
  • test_partial_index_sql
  • test_partial_index_json_roundtrip_trims_predicate

Fixes #30