feat: add Poe OAuth auth plugin

[kamilio]

Issue for this PR

Closes #18478

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds Poe as a built-in auth plugin. The plugin supports two auth methods: browser-based OAuth using PKCE (opens poe.com/oauth/authorize, exchanges the authorization code for an API key via api.poe.com/token) and manual API key entry. The implementation follows the same patterns as the existing Codex auth plugin (local callback server on an ephemeral port, CSRF state validation, PKCE challenge, timeout handling).

The plugin is registered in the INTERNAL_PLUGINS list in src/plugin/index.ts alongside the existing Codex, Copilot, and GitLab plugins.

How did you verify your code works?

• 12 tests pass covering: OAuth URL construction, browser open flow, error/missing-code/invalid-state callback handling, successful token exchange, expiry calculation, expired token re-login error, plugin provider visibility, ProviderAuth registration, and end-to-end model listing + chat stream with a mock server.
• bun typecheck passes.
• Full monorepo bun turbo typecheck passes (13/13 packages).
• Manual tests performing the OAUth Flow

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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