feat: allow opting out of text marshaling

[efegokdemir]

Summary

Types such as database null wrappers can implement encoding.TextMarshaler without representing all of their fields in the text form. Automatically selecting that interface can therefore lose data during round trips. This adds explicit opt-out controls while preserving the current default behavior.

Changes

• Add Encoder.UseTextMarshaler(false) to encode the underlying value instead of its text representation.
• Add Decoder.UseTextUnmarshaler(false) to decode the underlying value instead of invoking the text unmarshaler.
• Keep both interfaces enabled by default for compatibility.
• Add a regression test covering a struct whose text representation omits a field.

Testing

• go test ./... — passed
• go test -race ./... — passed
• go vet ./... — passed
• git diff --check — passed

Closes #11