refactor(postgresql): eliminate duplicate _splitPostgresStatements in favor of SqlStatementSplitter #1517

Summary

In lib/features/postgresql/postgres_table_utils.dart (lines 66–109):

isAllowedPostgresSelectQuery relies on _splitPostgresStatements(sql), which creates multiple full-text masked copies of the SQL string:

List<String> _splitPostgresStatements(String sql) {
  final masked = _maskPostgresComments(maskSqlLiteralsForLimitScan(sql));
  final statements = <String>[];
  var start = 0;
  for (var i = 0; i < masked.length; i++) {
    if (masked[i] == ';') {
      statements.add(sql.substring(start, i));
      start = i + 1;
    }
  }
...

This duplicates the functionality already provided by the core SqlStatementSplitter (lib/core/database/sql_statement_splitter.dart), while generating 2 full-text string buffers and multiple unnecessary substring slices.

Proposed Solution

1. Replace _splitPostgresStatements(sql) with SqlStatementSplitter.spans(sql).
2. Remove private helper functions _splitPostgresStatements and _maskPostgresComments in postgres_table_utils.dart.

Acceptance Criteria

• isAllowedPostgresSelectQuery uses SqlStatementSplitter.spans for statement boundary detection.
• Over 40 lines of duplicate masking code are deleted.
• All table browse SQL validation tests pass.