feat(gemini): ship gemini_oauth plugin + fix Code Assist streaming

[chucky5o]

What & why

model_factory.py already references code_puppy.plugins.gemini_oauth for the
gemini_oauth model type, but the plugin was never shipped — so that model
type is a dangling import today. This PR ships the plugin and fixes two
streaming bugs, letting users run Gemini models through their local
Gemini / Antigravity CLI's existing OAuth login — no API key.

Google has been rebranding the Gemini CLI tooling as "Antigravity". Both
write credentials to ~/.gemini/, so the same plugin handles either name.

Changes

1. New gemini_oauth plugin (code_puppy/plugins/gemini_oauth/)

Reuses the token the Gemini CLI stores at ~/.gemini/oauth_creds.json and calls
the Code Assist API (cloudcode-pa.googleapis.com). Exposes the three symbols
model_factory expects:

• GEMINI_OAUTH_CONFIG — Code Assist base URL + API version
• get_valid_access_token() — reads the CLI creds, auto-refreshes when expired, writes back chmod 600
• get_project_id() — discovers the managed GCP project via loadCodeAssist and caches it

Client credentials (the public installed-app values from the Gemini CLI) are read
from GEMINI_OAUTH_CLIENT_ID / GEMINI_OAUTH_CLIENT_SECRET env vars — no
secret is committed. The plugin README explains how to extract them from a
local gemini install to enable auto-refresh.

2. Fix streaming in gemini_code_assist.py

Two bugs broke the streaming path (which is what the agent runtime uses by default):

• request_stream() was missing the run_context parameter the runtime passes
  → TypeError: takes 4 positional arguments but 5 were given
• the streaming response class didn't inherit from pydantic_ai's StreamedResponse
  ABC → 'StreamedResponse' object has no attribute 'get'. Replaced with
  CodeAssistStreamedResponse, a proper @dataclass subclass implementing
  _get_event_iterator() (mirrors the existing GeminiStreamingResponse).

3. Docs

• docs/LOCAL_PROVIDERS.md — how to wire Ollama, Claude Code, and
  Gemini/Antigravity from their local CLIs, all switchable via /model.
• code_puppy/plugins/gemini_oauth/README.md — plugin setup + troubleshooting.

Testing

Verified end-to-end through the code-puppy CLI against a real account:

• /model antigravity-flash → live generateContent call returns a response
• token auto-refresh from an expired ~/.gemini/oauth_creds.json works
• managed project ID discovered via loadCodeAssist and cached
• non-streaming and streaming paths both exercised

Notes for the maintainer

• Personal model lists live in ~/.code_puppy/extra_models.json (not committed).
• If you'd prefer to hardcode the public client credentials rather than use env
  vars, that's a one-line change — I used env vars to keep secret scanners happy.

🤖 Generated with Claude Code

[mpfaffenberger]

Please make linters + tests pass and remove claude code authorship