docs: fix the streamable client snippet to use (*Client).Connect

[tonydzi]

What was broken

docs/protocol.md → Streamable Transport → client side shows:

client, err := mcp.Connect(ctx, transport, &mcp.ClientOptions{...})

There is no top-level mcp.Connect. The package has (*Client).Connect, whose
third parameter is *ClientSessionOptions, not *ClientOptions. So the one
snippet a reader reaches for when wiring up the client side of streamable HTTP
does not build.

It is also the only outlier in the docs: quick_start.md, client.md,
server.md, troubleshooting.md and protocol.md itself (lines 470, 538) all
already use client.Connect(ctx, transport, nil).

How I checked

Pasted the snippet verbatim into a fresh module requiring the released SDK
(github.com/modelcontextprotocol/go-sdk v1.7.0):

$ go build ./...
./broken.go:15:21: undefined: mcp.Connect

Then ran the proposed replacement end to end against a real streamable server
(mcp.NewStreamableHTTPHandler behind httptest.NewServer), connecting and
calling a tool over the session:

BUILD OK
tool result: {"greeting":"Hi you"}

What it is now

transport := &mcp.StreamableClientTransport{
	Endpoint: "http://localhost:8080/mcp",
}
client := mcp.NewClient(&mcp.Implementation{Name: "client", Version: "v1.0.0"}, nil)
session, err := client.Connect(ctx, transport, nil)

Change is in internal/docs/protocol.src.md; docs/protocol.md regenerated with
go generate ./internal/docs, both committed together. Docs-only, no code paths
touched.

---

Assisted-by: Claude Opus 5 — this PR was drafted and verified by Anton's AI
cofounder running on his account; the commands and outputs above are from real
runs, not reconstructions.