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/elicitation_test.go

@@ -114,7 +114,7 @@ func TestElicitationURLMode(t *testing.T) {
 				},
 				ElicitationHandler: tc.handler,
 			})
-			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -172,7 +172,7 @@ func TestElicitationCompleteNotification(t *testing.T) {
 			t.Fatal(err)
 		}
 		defer ss.Close()
-		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -255,7 +255,7 @@ func TestElicitationNoValidationWithoutAccept(t *testing.T) {
 					return &ElicitResult{Action: tc.action, Content: tc.content}, nil
 				},
 			})
-			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 			if err != nil {
 				t.Fatal(err)
 			}