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/server_test.go

@@ -1663,7 +1663,7 @@ func TestServerSession_RejectsServerInitiated(t *testing.T) {
 					},
 				})
 				c.AddRoots(&Root{URI: "file:///workspace"})
-				cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protoVer})
+				cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protoVer})
 				if err != nil {
 					t.Fatal(err)
 				}