Support three-state optional fields (Optional<T>) for PATCH serialization

[OmarAlJarrah]

Problem

PATCH and JSON Merge Patch operations need to distinguish three states for each field:

• absent — omit the field; leave the server value unchanged
• explicit null — send null; clear the server value
• present — send a concrete value

Plain nullable types (T?) collapse absent and null into a single representation, so models cannot express partial updates faithfully. The initial serialization layer intentionally ships without a dedicated type for this; consumers must hand-roll converters or fall back to JsonNode.

Proposed direction

Introduce a three-state Optional<T> value type (absent / null / present) with:

• a System.Text.Json converter, and
• source-generation support that omits absent members on write.

This is the .NET counterpart to the Tristate<T> container the Java and Python SDKs already provide.

Requirements & considerations

• Trim-safe and NativeAOT-compatible. The converter and the omit-on-absent behavior must work with source-generated JsonSerializerContexts — no runtime reflection.
• Omission mechanism. Skipping an absent member cannot be expressed through DefaultIgnoreCondition for a struct; it needs a JsonTypeInfo / ShouldSerialize modifier integrated with the source generator.
• Additive. This can land after the initial serialization work without changing existing public APIs.
• Ergonomics. Implicit conversion from T, factory helpers (Optional.Of(value), Optional<T>.Absent), and clean pattern matching.

Out of scope (for now)

• Full RFC 7386 (JSON Merge Patch) document handling, unless we choose to layer it on top of Optional<T>.