Add connection-scoped query resolution

[fsistemas]

Summary

Adds core support for connection_queries config parsing and query resolution.

• Adds validated top-level connection_queries config support.
• Resolves named queries in the agreed order:
  1. connection_queries.<connection>.<query_name>
  2. queries.<query_name>
  3. existing raw SQL / @file.sql handling
• Preserves existing global-only config behavior.
• Keeps CLI JSON-on-stderr error envelope for malformed scoped-query config.

Verification

• .venv/bin/pytest tests/test_sql2json.py::test_run_query_by_name_uses_connection_scoped_query tests/test_sql2json.py::test_run_query_by_name_falls_back_to_global_query tests/test_sql2json.py::test_run_query_by_name_scoped_query_takes_precedence_over_global tests/test_sql2json.py::test_run_query_by_name_rejects_malformed_connection_queries tests/test_sql2json.py::test_run_query_by_name_rejects_connection_queries_for_unknown_connection tests/test_cli.py::TestErrorOutput::test_malformed_connection_queries_stderr_is_json — 6 passed
• .venv/bin/pytest — 133 passed, 12 deselected
• .venv/bin/black --check sql2json tests — 16 files would be left unchanged
• .venv/bin/flake8 sql2json tests — passed
• .venv/bin/mypy sql2json — Success: no issues found in 5 source files