Ack 1.4.0

Ack 1.4.0

Ack 1.4 adds @Optional(), @Required(), and @NotNull() so class-first
models can override JSON key presence independently of whether a present
value may be null. @AckField(presence: ...) remains during migration
and is deprecated for removal in 2.0.0.

• Keep omit-vs-required separate from JSON null acceptance.
• Reject a present JSON null on a nullable Dart field with @NotNull().
• Generate copyWith from the constructor type so optional non-nullable
  codec fields compile and do not receive null.

All six packages use version 1.4.0: ack, ack_annotations, ack_generator,
ack_firebase_ai, ack_json_schema_builder, and ack_mcp_dart.
The runtime, Firebase, JSON Schema, and MCP packages have no runtime or
public API changes from 1.3.0.