fix(spec): prefer application/a2a+json in HTTP binding (#1753)

## Summary
- Updates HTTP+JSON/REST binding guidance to state that
`application/a2a+json` SHOULD be used for request/response payloads.
- Aligns HTTP A2A examples in the spec to use `Content-Type:
application/a2a+json`.
- Keeps JSON-RPC-specific `/rpc` example on `application/json` to avoid
changing JSON-RPC semantics.

## Why
Issue #1748 identified inconsistent guidance: Section 11.1 said
`application/json`, while many HTTP examples and the registered A2A
media type (`application/a2a+json`) already pointed to A2A-specific
media typing.

This change resolves that inconsistency and improves interoperability
clarity for HTTP+JSON implementations.

Fixes #1748

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>