Detach cli lifespan from the context passed to Client.Start

[qmuntal]

Fixes #668

[Copilot]

Pull request overview

This PR fixes Go SDK issue #668 by ensuring the Copilot CLI subprocess spawned by Client.Start(ctx) is not tied to (and killed by) cancellation of the provided context after startup.

Changes:

• Spawn the CLI process with exec.Command instead of exec.CommandContext so canceling the Start context doesn’t kill the CLI.
• Thread the caller context through ensureConnected(ctx) and simplify foreground session methods to reuse that helper.
• Add a regression test asserting that canceling the Start context does not break subsequent CLI requests (Ping).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
go/client.go	Decouples CLI process lifetime from Start context; refactors connection checks and tightens startup/dial timeouts.
go/client_test.go	Adds regression test ensuring Start-context cancellation doesn’t kill the CLI process.