[Bug]: Contradictory A2A-Version transmission requirements

### What happened?

The specification says:

[3.6.1 (Client Responsibilities)](https://a2a-protocol.org/latest/specification/#361-client-responsibilities)
> Clients MUST send the A2A-Version header with each request [...]

But just a bit later, it says:
> Clients MAY provide the A2A-Version as a request parameter instead of a header.

The specification for JSON-RPC Protocol Binding adds:

[9.2 (Service Parameter Transmission)](https://a2a-protocol.org/latest/specification/#92-service-parameter-transmission)

> A2A service parameters defined in [Section 3.2.6](https://a2a-protocol.org/latest/specification/#326-service-parameters) MUST be transmitted using standard HTTP request headers, as JSON-RPC 2.0 operates over HTTP(S).

Same for HTTP+JSON/REST Protocol Binding:
[11.2 (Service Parameter Transmission)](https://a2a-protocol.org/latest/specification/#112-service-parameter-transmission)
> A2A service parameters defined in [Section 3.2.6](https://a2a-protocol.org/latest/specification/#326-service-parameters) MUST be transmitted using standard HTTP request headers.

Per [RFC2119](https://datatracker.ietf.org/doc/html/rfc2119), MUST means that sending the header "is an absolute requirement of the specification".

It should be clarified which requirement takes precedence in which case.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Read the specification sections linked in the issue: 3.6.1, 9.2, and 11.2. Understand the difference between 'MUST' and 'MAY' per RFC2119. The task is to propose a clarification in the specification text, likely in the A2A documentation repository. Check if there are existing pull requests or issues about this contradiction. The outcome is a suggested edit to resolve the ambiguity.

索引モデルが issue の本文から書いたものです。