docs: fix non-compiling Rust external-transport example

[examon]

The Rust example under "Connecting to an external CLI server" in docs/getting-started.md constructed Transport::External { host, port } and omitted the variant's third required field, so the documented example did not compile. This adds connection_token: None,.

Fixes #2141

Why None

• The server the section tells you to start has no token: copilot --headless --port 4321, which on startup reports No COPILOT_CONNECTION_TOKEN was set, so connections will be accepted from any client.
• None of the sibling language tabs in the same example pass a token.
• The only other Rust example that constructs this variant, in docs/setup/multi-tenancy.md, already passes connection_token: None.

Before and after

Against the published crate, installed the way the guide instructs (cargo add github-copilot-sdk --features derive, which resolves to 1.0.8), with the example pasted into an async main as the guide's own complete examples do:

Before:

$ cargo build
error[E0063]: missing field `connection_token` in initializer of `github_copilot_sdk::Transport`
 --> src/main.rs:9:25
  |
9 |     options.transport = Transport::External {
  |                         ^^^^^^^^^^^^^^^^^^^ missing `connection_token`

error: could not compile `copilot-demo` (bin "copilot-demo") due to 1 previous error

After:

$ cargo build
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.62s

The fixed example was also run against a real copilot --headless --port 4321 server: it connects and creates a session, and fails with ConnectionRefused once the server is stopped, so the connection is genuinely going to the external server.

Checks

• Every Rust fence in docs/getting-started.md was compiled before and after. This change removes the file's only compile error and introduces none.
• just validate-docs gives the same per-language results before and after; the only difference is a one-line shift in generated block metadata.
• No Rust source changed, so the Rust build, formatting and lint results are identical to base.

No regression test

There is no test to add: scripts/docs-validation/extract.ts has no rust entry in LANGUAGE_MAP, so Rust fences under docs/ are never extracted and no validator covers them. That is also why this drifted unnoticed. Adding Rust coverage needs a validator and a workflow job, so I have left it out of this change.

Out of scope

Noted while here, kept separate: the TypeScript tab in this same section is separately stale, as it passes cliUrl, which is no longer a CopilotClientOptions field.

[Copilot]

Pull request overview

Fixes the Rust external CLI transport example so it compiles with the current SDK API.

Changes:

• Adds the required connection_token field with None.

Show a summary per file

File	Description
docs/getting-started.md	Corrects the Rust external transport example.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Medium

[SteveSandersonMS]

Reviewed for scope, correctness, cross-language consistency, and manual Rust API validation. The one-line documentation fix is necessary, correct, and minimal.