mssql_python: discrete connection fields are silently ignored when connection_string is set

Start in sqlspec/adapters/mssql_python/core.py at build_connection_config, then compare the mssql_python behavior with the arrow_odbc adapter's connection configuration logic and documented precedence. Use the reproduction in the issue to verify that database overrides are no longer silently ignored, and add or update coverage for the chosen conflict behavior.

Summary

In the mssql_python adapter, build_connection_config returns early when connection_string is present, so every discrete field in the same connection_config (server, database, uid, pwd, encrypt, extra, ...) is dropped without a warning or error.

# sqlspec/adapters/mssql_python/core.py
connection_string = config.pop("connection_string", None)
if connection_string is not None:
    return str(connection_string), connect_kwargs

Why it matters

A common pattern is to hold one base config and derive per-database configs from it by overriding database:

base = {"connection_string": "Server=host,1433;UID=app;PWD=...;Encrypt=yes;"}
per_db = MssqlPythonConfig(connection_config={**base, "database": "sales"})

per_db connects to the login's default database, not sales. Nothing fails — queries simply run against the wrong database, which is hard to notice when iterating over several databases on one instance.

Reproduction

from sqlspec.adapters.mssql_python.core import build_connection_config

conn_str, _ = build_connection_config(
    {"connection_string": "Server=host;UID=u;PWD=p;", "database": "sales"}
)
assert "sales" in conn_str  # fails: "Server=host;UID=u;PWD=p;"

Expected

Either of these would remove the silent wrong answer:

1. Merge discrete fields with the explicit string, with a documented precedence. The arrow_odbc adapter already does this (its build_connection_config composes the discrete fields and appends connection_string as a suffix, with the precedence described in the docstring), so the two ODBC-style adapters would behave the same way.
2. Raise a ValueError (or ImproperConfigurationError) when connection_string is combined with any connection-string-level field, naming the conflicting keys.

Option 1 seems the friendlier of the two, provided the precedence is stated (an explicit field overriding the same key inside the string is the least surprising for the per-database case above).

Version

sqlspec 0.63.1; still present on main.