Option 3: Null-contract proxy design for ModelReaderWriter

[m-redding]

Option 3: Null-contract proxy design (no DiscriminatorRouter)

This PR explores the simplest alternative design for the ModelReaderWriter proxy/routing feature.

Comparison of all three approaches

Aspect	Option 1 (PR #59083)	Option 2 (PR #59313)	Option 3 (this PR)
Core type	ModelProxy (generic, IS-A model)	DiscriminatorRouter (non-generic abstract class)	No new type
Registration	AddProxy	AddProxy + AddDiscriminatorRouter	AddProxy only
Write resolution	Last registered proxy	Last registered proxy	Last in list
Read resolution	Routers then proxy then model	Routers (FIFO, CanHandle) then proxy then model	List (FIFO), first non-null Create wins, then model
Discriminator pattern	Explicit CanHandle methods	Explicit CanHandle + Create methods	Convention: return null from Create if can't handle
New public types	ModelProxy	DiscriminatorRouter	None
Generic variance	Blocked (invariance)	Solved (non-generic router)	N/A (no router type)

API Surface (Option 3)

// Registration - both write and read use the same list
options.AddProxy<T>(IPersistableModel<T> proxy);
options.AddProxy<T>(IJsonModel<T> proxy);

// Resolution (write)
options.ResolveProxy<T>(IPersistableModel<T> model);  // returns last in list
options.ResolveProxy<T>(IJsonModel<T> model);          // returns last in list

The null contract

The discriminator case is handled purely by convention:

• Register multiple proxies via AddProxy
• On read, proxies are consulted first-to-last
• Each proxy's Create returns null if it can't handle the data
• The first proxy to return non-null wins
• If all proxies return null, the model itself handles deserialization

Tradeoffs

Pros:

• Simplest API - no new public types at all
• No conceptual split between proxy and router
• Single registration mechanism (AddProxy)
• Easy to understand: return null = can't handle, return value = handled

Cons:

• Null-returning from Create is a convention, not enforced by the type system
• No explicit CanHandle - must call Create to discover if a proxy can handle
• Less discoverable - return null isn't as self-documenting as overriding CanHandle
• List semantics mean order matters (same as Option 2 routers)

[m-redding]

TODO comment is outdated

[m-redding]

TODO - honestly not 100% sure exactly what we want for this, if someone wants to just add some proxies for reading only, I guess they would need to manually delegate to the base model in the write path?