mcp: decouple HTTPClient concrete implementation from client transports

[manuelibar]

Description

This change decouples client transports from the concrete net/http.Client by accepting any HTTP client that implements Do(http.Request) (http.Response, error). This enables middleware (auth, tracing, retries), improves testability, and unifies behavior across streamable and SSE transports.

What changed

• Introduce HTTPClient interface in the mcp package.
• StreamableClientTransport: change HTTPClient field type to the interface; default to http.DefaultClient when nil.
• SSEClientTransport: same change as streamable.

Why

• Flexibility to inject custom HTTP clients with middleware.
• Better testability via interface-based dependencies.
• Consistency across transports (streamable and SSE).

Compatibility

• Backward compatible: existing code passing http.Client continues to work (it satisfies the interface).
• No behavior changes; defaults remain the same when HTTPClient is nil.

Testing

• go test ./... passes locally.
• Existing transport tests validate streamable/SSE flows.

Fixes #522

[manuelibar]

@findleyr I'll close this PR given that the proposal was declined.
Thanks!