fix: add Conn and *Conn to mapperFor and isUnsafe type switches

[xbrxr03]

Problem

mapperFor() and isUnsafe() only handle DB, *DB, Tx, and *Tx types in their type switches. When a Stmt is created from a *sqlx.Conn, both functions fall through to the default case:

• mapperFor(conn) returns the global mapper instead of conn.Mapper
• isUnsafe(conn) returns false instead of conn.unsafe

This silently drops any custom Mapper configured on a Conn (e.g. via tagMapFunc), causing field mapping to use the wrong mapper when creating prepared statements from a connection.

Reported in #975.

Fix

Add Conn and *Conn to both type switches:

// mapperFor
case Conn:
    return i.Mapper
case *Conn:
    return i.Mapper

// isUnsafe
case Conn:
    return v.unsafe
case *Conn:
    return v.unsafe

Tests

• TestMapperForConn: verifies mapperFor returns the correct Mapper for Conn and *Conn, including custom mapper instances
• TestIsUnsafeConn: verifies isUnsafe returns the correct unsafe flag for Conn and *Conn

Fixes #975