[Katyara11]

Summary

Closes the last sharp edge in the AOT story: a type your contracts serialize that your
JsonSerializerContext does not register is now RXH024 at build time, instead of a resolver
error on the first call that needs it in a deployed trimmed application.

The generator already knows the complete type set, so you no longer keep the context in step by
hand.

Changes

• RXH024 (warning) — reports a serialized type no JsonSerializerContext registers. The
  message names the type, the method, the exact [JsonSerializable] line, and the context.
• src/Runax.Http.SourceGenerator/JsonContextCoverage.cs — the type collection, the context scan
  and the comparison, plus the recorded reason the context cannot simply be emitted.
• A second, compilation-wide pipeline in ClientGenerator — the check joins every contract's types
  against every context, which no per-interface analysis can see.
• 16 tests; docs in aot.md, serialization.md, diagnostics.md, package README, CHANGELOG.

Type

• Feature
• Bug fix
• Refactor
• Chore (CI, deps, config)

Checklist

• Code builds without errors
• Tests added/updated

Notes for review

Why this is a diagnostic and not a generated context

I verified the constraint instead of assuming it, and the answer is more precise than "generators
cannot see each other's output":

Registration	Visible to System.Text.Json's generator?
RegisterPostInitializationOutput	Yes — compiles, populated, serializes at run time
RegisterSourceOutput (derived from user code)	No — CS0534, abstract members unimplemented

Post-initialization output joins the compilation every generator sees; analysis-derived output does
not. The type set can only come from analysing the contracts, which means RegisterSourceOutput —
so emitting the context is impossible, not merely awkward. Recorded in docs/aot.md and on
JsonContextCoverage so nobody retries it.

Two deliberate blind spots

A diagnostic that cries wolf gets suppressed wholesale and then protects nobody, so both of these
err toward silence:

• Bare primitives and string are never reported. A probe showed a context registering a
  single record resolved its int and string property types for free, while bool and every
  collection came back null. So nothing is unconditionally free — but those are overwhelmingly
  likely to be, and a method whose body or response is a bare primitive is rare.
• Any context counts. Nothing at compile time can tell which context is wired to which client,
  and several may be. Taking the union is the only choice that cannot invent a false positive.

Warning, not error

A mixed setup — one client on a source-generated context, another deliberately on the
reflection-based serializer — is legitimate, and an error would fail that build with no recourse
but suppression. Under warnings-as-errors it fails the build anyway, which is how the AOT smoke
test guards its own context: deleting a [JsonSerializable] from AotJsonContext now fails the
build, which I confirmed before restoring it.

Collections are the gap worth catching

Registering Repository does not cover IReadOnlyList<Repository> — confirmed by probe. A
method returning Task<IReadOnlyList<T>> is exactly where someone forgets, and it is covered.

Implementation note

The type set is read off the existing ClientModel rather than re-walking symbols, so the
generator keeps one understanding of what each return shape serializes. A second walk would drift
the first time a return shape is added. ClientGenerator edits are confined to Initialize,
AnalysisResult and one return statement, to keep the rebase against the parallel query-binding PR
mechanical.

Verification

1527 tests across net8.0/net9.0/net10.0; --warnaserror clean, dotnet format clean, AOT publish
with zero trim warnings and the binary runs. All doc links and anchors resolve.

Rebased onto main twice while open, since #5 and #6 landed first. #6 claimed RXH022 and
RXH023, so this diagnostic is RXH024 — ids are never reused, so the renumber rather than a
shared number is the right resolution. The two generator changes touch the same files but not the
same code paths, and the combined suite is green.