Prevent auth acceptance tests from opening browser tabs

[Fab10-CircleCi]

Summary

Acceptance tests were launching real browser tabs because the interactive auth UI models called browser.OpenURL directly.

This adds an injectable browser opener to the login and signup flow models. In normal usage, the opener still defaults to browser.OpenURL, so production behavior is unchanged. In acceptance tests, TestEnv now sets CIRCLECI_NO_BROWSER_OPEN=true, which makes the command layer pass a no-op opener instead.

CIRCLECI_NO_BROWSER_OPEN is intentionally undocumented; it is a test seam, not a supported user-facing configuration flag.

Context

auth login already opened a browser tab during the interactive browser-login path. PR #1310 added signup coverage that exercises browser auth more often, so running acceptance tests became noisy and disruptive locally.

Before this change, the login interactive path, signup start, and signup-to-login fallback could all open browser tabs during acceptance tests.

With this change:

• Normal circleci auth login and circleci auth signup still open the browser.
• Acceptance tests still exercise the full interactive login/signup flows.
• Acceptance tests no longer open real browser tabs.

Testing

Unit tests cover the browserOpener() env-var switch and the signup model's use of the injected opener with and without NoBrowser.

```sh
go test ./internal/ui ./internal/cmd/cmdauth
go test ./acceptance -run 'TestAuthSignup|TestAuthLogin' -count=1
go test ./acceptance -count=1
go test ./...
task ci:check
```

🤖 Generated with Claude Code

[pete-woods]

Resolved the issues with signup on the backend