fix(gpt-account): relay OAuth login prompts through the extension UI

[code-yeongyu]

Summary

Closes #1485.

/gpt-account add relayed every OAuth AuthPrompt to ctx.ui.input(prompt.message), so the provider's select prompt (Select OpenAI Codex login method:) rendered as an empty text field and an empty Enter reached openai-codex as Unknown OpenAI Codex login method:. The device-code flow printed the verification URL without the user code, the browser flow never opened a browser, and the manual-code dialog ignored the provider's per-prompt abort signal.

What changed

• New builtin/oauth-login-interaction.ts — createExtensionLoginInteraction(ctx, { providerLabel, openBrowser? }):
  • select → ctx.ui.select(message, labels, { signal }), chosen label mapped back to the option id;
  • text / secret / manual_code → ctx.ui.input(message, placeholder, { signal });
  • dialog signal = command signal + per-prompt AuthPrompt.signal (AbortSignal.any), so the paste-the-code dialog closes when the local callback server wins; dismissed/aborted → Login cancelled;
  • auth_url → openBrowser(url) when ctx.mode === "tui" (same as the /login dialog) + notice with the URL and instructions;
  • device_code → notice with the verification URL and Enter code: <userCode>; info prints links; progress prints the message.
• builtin/gpt-account.ts and claude-sdk-oauth/account-command.ts addAccount use the shared interaction; both factories accept an optional openBrowser dep for tests.
• Tests: test/suite/account-command-harness.ts (records ui.select/ui.input calls; unanswered input resolves "" like the reporter's empty Enter), account-extension.test.ts (/account), gpt-account-extension.test.ts (list/pin/remove), gpt-account-add.test.ts (the 6 new relay cases + 3 existing add cases). RED before the fix: 5 failed / 14 passed; GREEN after: 36 passed across the account, claude-account, rpc-login and app-server-account suites.
• Trackers: builtin/changes.md, claude-sdk-oauth/changes.md, packages/coding-agent/CHANGELOG.md [Unreleased] → Fixed.

Verification

• npx vitest --run test/suite/{account-extension,gpt-account-extension,gpt-account-add,claude-sdk-oauth-account-command,rpc-login-interactive-prompts,app-server-account}.test.ts → 36 passed
• npx tsc --noEmit -p tsconfig.json → clean
• node scripts/check-pr-changelog.mjs --base $(git merge-base HEAD origin/main) → PASS
• Real TUI run of /gpt-account add from the built dist/cli.js: evidence will be attached to [Bug]: /gpt-account add renders the OpenAI Codex login-method select as an empty text input (Unknown OpenAI Codex login method:) #1485 (selector with Browser login (default) / Device code login (headless)).

Notes

• Browser auto-open is TUI-only on purpose: an RPC/desktop client renders the notice on its own machine, and the server never opened a browser for /login either.
• Discord report: https://discord.com/channels/1452487457085063218/1490539106797621259/1546781611930951741

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Summary by cubic

Fixes /gpt-account add and /claude-account add so OAuth login prompts render as their proper UI instead of every prompt being sent to a plain text input. The login-method chooser now shows as a real selector, the device-code flow prints the user code, the browser flow opens the browser in the TUI, and the manual-code dialog closes when the provider's callback completes the login. Closes #1485.

• Adds a shared oauth-login-interaction.ts used by both account commands that routes prompts by type: select to ui.select, text/secret/manual_code to ui.input with the placeholder, and auth_url/device_code/info to notices.
• Binds every dialog to the per-prompt AuthPrompt.signal so a dismissed or aborted prompt rejects with Login cancelled.
• Extracts a test harness recording ui.select/ui.input calls, with new coverage for the relay cases in gpt-account-add.test.ts.

^{Written for commit eb1297d. Summary will update on new commits.}