## Motivation and Context
`MCP::Client::HTTP` builds its Faraday connection internally in a private `client` method
with no way to customize the middleware stack or adapter.
Users who need observability (request/response recording, failure logging) or
a different HTTP adapter must override a private method, coupling to an internal API.
This accepts an optional block in `MCP::Client::HTTP.new` that yields the Faraday builder
after default middleware is configured, allowing users to add custom middleware or swap
the HTTP adapter.
## How Has This Been Tested?
Added a test that verifies custom headers set via the block are included in requests.
All existing tests continue to pass.
## Breaking Changes
None. The block argument is optional, so existing code is unaffected.
Resolves#303
- Set Accept header to request JSON or SSE responses
- Validate response Content-Type and raise error for non-JSON responses
- Add Accept header to test fixtures
- Use explicit hash syntax for Ruby 2.7 compatibility
