fix: don't treat @ inside a string literal as a named parameter in Select

[makoto-developer]

Description

Fixes #7235

When a string passed to Select contains an @, Select treats the whole statement as a named expression (clause.NamedExpr) and consumes the following arguments as its variables. When those arguments are plain column names, they are silently dropped:

DB.Table("users").Select("name = 'test@example.com' as is_example", "age")
// before: SELECT name = 'test@example.com' as is_example FROM `users`   (age is gone)
// after:  SELECT name = 'test@example.com' as is_example,age FROM `users`

The @ here is part of a string literal, not a named-parameter placeholder.

Fix

Skip the named-expression path when every extra argument is a plain column name (string or []string); in that case a bare @ is just part of the SQL text and the arguments are treated as additional columns, matching the []string form of Select.

Passing actual named arguments (sql.Named, maps, structs) still takes the named-expression path and is unaffected — the existing COALESCE(age, @default) test continues to pass.

Testing

Added TestSelectWithAtInStringLiteral (fails before, passes after). go test ./... passes for both the root module and the tests module; golangci-lint reports no new issues; gofumpt/go vet clean.