Fix Column.string returning raw bytes for non-text column types

[ericraio]

Fix Column.string returning raw bytes for non-text column types

Motivation:

The DataStax C driver's cass_value_get_string has no type checking - it calls as_string_ref() and returns CASS_OK for any non-null value, regardless of the column's actual CQL type.

The Swift toString() helper called this unconditionally, meaning Column.string appeared to succeed for int, bigint, blob, uuid, boolean, float, double, UDT and frozen collection columns, returning their raw binary serialization decoded as lossy UTF-8.

This caused callers that try Column.string before falling back to typed accessors to silently receive binary garbage instead of nil, breaking display layers for frozen UDTs and complex types.

Modifications:

• In toString(), call cass_value_type first and return nil early for any type that is not ASCII, TEXT, or VARCHAR.
• Replace String(decoding:as:UTF8.self) (non-failable, silently replaces invalid UTF-8 with U+FFFD) with String(bytes:encoding:) (failable, returns nil for invalid UTF-8).
• Add DataTests.swift with integration tests verifying Column.string returns a value for text-typed columns and nil for all other types.

Result:

Column.string now returns nil for non-text columns, matching the documented semantics. Typed accessors on those columns continue to work correctly. Callers that use Column.string as a fallback for unknown types will correctly fall through to bytes or other representations.