mcp: Implement support for SEP-2575 on client side (#975)

Implements the client half of the SEP-2575 / `2026-06-30` protocol. This
CL is client-only; server-side `server/discover` is still a
`MethodNotFound` stub.
### Changes
- **`server/discover` probe in `Client.Connect()`** (`client.go`). When
the caller opts into `protocolVersion >= 2026-06-30`, the client probes
the server with `server/discover` before falling back to the legacy
`initialize`
  handshake.
- **Per-request `_meta` injection** (`shared.go`, `injectRequestMeta`).
For`>= 2026-06-30` sessions every outgoing request gets the SEP-2575
triple (`protocolVersion`, `clientInfo`, `clientCapabilities`).
- **Server-side fix**: `ServerSession.handle` no longer pre-populates
`state.InitializeParams` from a `server/discover` request's `_meta`, so
a subsequent fallback `initialize` is not rejected as a duplicate.
### Fallback rules
Client falls back to the legacy `initialize` handshake when discover
returns:
- `MethodNotFound` (-32601),
- `UnsupportedProtocolVersionError` (-32004),
- an HTTP `Bad Request` (substring-matched on the error message, to
interop
  with pre-2026-06-30 servers).

Fixes #966

mcp/shared.go

@@ -105,6 +105,7 @@ func defaultSendingMethodHandler(ctx context.Context, method string, req Request
 		// capabilities, so any panic here is a bug.
 		params = initParams.toV2()
 	}
+
 	// Notifications don't have results.
 	if strings.HasPrefix(method, "notifications/") {
 		return nil, req.GetSession().getConn().Notify(ctx, method, params)
@@ -344,6 +345,9 @@ func clientSessionMethod[P Params, R Result](f func(*ClientSession, context.Cont
 
 // MCP-specific error codes.
 const (
+	// CodeUnsupportedProtocolVersion is the JSON-RPC error code defined by
+	// SEP-2575 for UnsupportedProtocolVersionError.
+	CodeUnsupportedProtocolVersion = -32004
 	// CodeHeaderMismatch indicates that HTTP headers do not match the corresponding values
 	// in the request body, or that required headers are missing or malformed.
 	CodeHeaderMismatch = -32001