feat(rest): serve HTTP+JSON responses as application/a2a+json - #1274

What

Spec 11.1: "Content-Type: application/a2a+json SHOULD be used for requests and responses". The spec uses it in every worked HTTP+JSON example - 20-plus of them.

Every REST response inherited Starlette's JSONResponse.media_type default of application/json. application/a2a+json appeared nowhere in the repo, so the constant is new.

Worth knowing: nobody implements this

All four SDKs with published ACTS metrics - python, go, java, rust - fail REST-CT-001. It is a SHOULD, so it blocks no one's conformance, and a reasonable response to this PR is "not yet, let the others move first". Merging it makes a2a-python the first. Flagging it rather than burying it, since the other six PRs in this stack all had at least one SDK already passing to corroborate them.

Two limits, both deliberate

Success responses only. The error path runs through rest_error_handler, which compat/v0_3/rest_adapter.py shares. 11.1 is the 1.0 binding's requirement, and v0.3 predates the media type, so changing that decorator would alter a different protocol version's wire format. Splitting it would mean making a bare decorator parameterized and touching every call site in both bindings - not worth it for a SHOULD the corpus does not test on the error path. The result is that 1.0 errors still answer application/json; worth a follow-up, not worth bundling here.

Responses only, not requests. 11.1 says both, but changing what the client sends risks servers that only accept application/json, and no test covers it.

No JSONResponse subclass. It would have reduced nine sites to an import swap, but rest_dispatcher.py aliases JSONResponse = Any when Starlette is absent, and class X(Any) is a TypeError on Python 3.10, which this package supports. Passing media_type= explicitly keeps the no-Starlette path importable and the sites greppable.

Result

Clears REST-CT-001 (should, rest only). With it, all three bindings are at zero failures at every level:

The remainder are skips whose preconditions the fixture does not meet, not failures.