Prefer verification_uri_complete for Copilot login

[skhe]

Fixes #738

This updates the Copilot device login flow to prefer verification_uri_complete over verification_uri when opening the browser.

Why:

• GitHub device flow can return a complete verification URL that already carries the user code
• CodexBar currently ignores that field and always opens the bare verification page
• In practice, this can make the Copilot login flow confusing because the user code is not reliably visible in the browser flow

Changes:

• Decode verification_uri_complete in CopilotDeviceFlow.DeviceCodeResponse
• Add a small verificationURLToOpen helper with fallback to verification_uri
• Use that helper in CopilotLoginFlow
• Add tests for both the preferred and fallback paths

Verification:

• swift build
• swift test --filter CopilotDeviceFlowTests

Notes:

• pnpm check could not complete in my environment because the lint script attempted to download SwiftFormat and failed before running the checks.

[steipete]

Landed via temp rebase onto main.

• Gate: swiftformat Sources Tests; swiftlint --strict; pnpm check; swift build; swift test -q; ./Scripts/compile_and_run.sh
• Land commit: ef86cda
• Merge commit: d968dd9

Thanks @skhe!