mcp: export ClientSessionOptions.ProtocolVersion (#1127)

## Summary

`ClientSessionOptions.protocolVersion` was unexported and documented as
test-only, so external consumers of `Client.Connect` could not request a
specific protocol revision.

This exports the field as `ProtocolVersion` with the same semantics:

- empty → request the latest supported version (unchanged default)
- non-empty → send that version in initialize/discover
- existing negotiate-down path is unchanged

## Usage

```go
cs, err := client.Connect(ctx, transport, &mcp.ClientSessionOptions{
    ProtocolVersion: "2025-11-25",
})
```

## Testing

- `go test ./mcp/ -count=1` (pass)

Fixes #1113

Co-authored-by: arimu1 <19286898+arimu1@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/mrtr_test.go

@@ -312,7 +312,7 @@ func TestMultiRoundTrip_ServerMiddleware(t *testing.T) {
 				},
 			})
 			c.AddRoots(&Root{URI: "file:///workspace", Name: "workspace"})
-			cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+			cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 			if err != nil {
 				t.Fatalf("client.Connect() error = %v", err)
 			}
@@ -516,7 +516,7 @@ func mustConnect(t *testing.T, s *Server, clientOpts *ClientOptions) *ClientSess
 	})
 
 	c := NewClient(testImpl, clientOpts)
-	cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client.Connect() error = %v", err)
 	}