Implement SQL Server 2025 Parity Features (Group 1)

[celsowm]

Implemented two major groups of SQL Server 2025 parity features:

1. Index and Constraint Enhancements: Added CLUSTERED, NONCLUSTERED, and FILLFACTOR support. Updated the SchemaExecutor to honor these flags during table creation and index rebuilding.
2. Logic and Expression Refinements: Added COALESCE, NULLIF, and LIKE ... ESCAPE. The LIKE matching now uses a dynamic programming approach to correctly handle escaped wildcards.
3. Session and Identity Management: Implemented SET IDENTITY_INSERT enforcement in the MutationExecutor and wired the IDENTITYCOL keyword into the identifier resolution logic.
4. Type System Parity: Enhanced parse_data_type to handle ISO-standard synonyms like CHARACTER VARYING and NATIONAL prefixes.

The catalog::ColumnDef and ast::ColumnSpec structures were expanded to include collation and is_clustered fields, with all relevant initializers updated across the engine. All changes are verified with new parity tests in tests/sql_server_2025_parity.rs.

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PR created automatically by Jules for task 3615242221267019679 started by @celsowm

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