feat(ws): add statement SQL reconstruction

[openexw]

Summary

• Add ws/unified.Stmt.ToSQL() to reconstruct executable SQL from stmt2 data bound after Prepare.
• Support parameterized queries, fixed-table inserts, and dynamic subtable inserts using insert into ? using ... tags(...) values(...).
• Support multi-table and multi-row inserts, tags, string escaping, NULL, VARBINARY/BLOB, and DECIMAL serialization; return an
  explicit error for GEOMETRY.
• Keep ToSQL() non-destructive so callers may still invoke Exec() afterward.
• Add table-driven unit tests for valid SQL, statement state errors, invalid bound data, invalid SQL templates, and SQL length
  limits.

Test Plan

• go test ./ws/unified -run '^TestStmtToSQL' -count=1

Notes

• ToSQL() supports only the new Bind API. Deprecated compatibility binding APIs do not preserve the original stmt2 types
  and return an explicit error.
• SQL reconstruction reuses the existing common.InterpolateParams implementation; this change only adds the adapter from
  stmt2 column-oriented bind data to row-oriented VALUES tuples.