std: support prepared SELECT/DDL via database/sql; add examples/tests

[theboringhumane]

Implement driver-side prepared statements for non-INSERT queries in the database/sql compatibility layer.

Changes:

• Introduce stdStmt and wire it into PrepareContext for non-INSERT statements

• Implement StmtQueryContext/StmtExecContext using underlying connection

• Preserve INSERT behavior via stdBatch (Exec-only)

• Add example examples/std/prepared.go and run it in examples suite

• Add tests: prepared select, USE + prepared select, and merged funds temp test

• Remove temporary test file; fix minor linter warning in clickhouse_std.go

Rationale:

• Restores database/sql compatibility for prepared read queries (e.g., Stmt.QueryContext) while keeping batch INSERT semantics

Summary

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[CLAassistant]

All committers have signed the CLA.

[kavirajk]

@theboringhumane thanks for the contribution. can you take care of the failing test cases?

code: 62, message: Syntax error: failed at position 58 (end of query) (line 3, col 5): . Expected one of: token, Dot, OpeningRoundBracket, FROM INFILE, SETTINGS, VALUES, FORMAT, SELECT, WITH

[theboringhumane]

@theboringhumane thanks for the contribution. can you take care of the failing test cases?

code: 62, message: Syntax error: failed at position 58 (end of query) (line 3, col 5): . Expected one of: token, Dot, OpeningRoundBracket, FROM INFILE, SETTINGS, VALUES, FORMAT, SELECT, WITH

I'll take care of that. No sweat.