feat: add opt-in "Follow redirects" connection flag

[maxdrift]

First off, thanks for maintaining this integration — what drew me to it is the ability to point it at any OpenAI-compatible host (TheGrid.ai, homelab gateways, etc.) rather than being locked to OpenAI/OpenRouter.
Small opt-in contribution back; happy to iterate on feedback.

Summary

Adds an opt-in Follow redirects flag to the integration's connection
settings so users behind a reverse proxy, gateway, or SSO front-end can
talk to OpenAI-compatible hosts that issue an HTTP 3xx before reaching
the actual API.

Discovered while testing this integration with TheGrid.ai, which performs a 307 redirect (within the same host)

Problem

Home Assistant's shared httpx client (get_async_client) disables redirect
following by default (follow_redirects=False), whereas the OpenAI SDK's
own default client follows redirects. Because this integration reuses HA's
shared client, any host that responds with a redirect surfaces the redirect
body as a connection error instead of following it:

Solution

• Add CONF_FOLLOW_REDIRECTS / DEFAULT_FOLLOW_REDIRECTS (default False).
• When enabled, get_authenticated_client builds the client with
  create_async_httpx_client(hass, follow_redirects=True) instead of
  get_async_client(hass). This keeps HA's shared client lifecycle
  (automatic cleanup) while opting into redirect following. Applies to both
  the Azure and non-Azure branches.
• Thread the flag through __init__.py (entry data) and config_flow.py
  (user step), and allow updating it at runtime via the change_config
  service.
• Surface the flag in the UI (strings.json, translations/en.json,
  services.yaml) and document it in docs/configuration.mdx.

Default is OFF, so existing users see no behavioral change — this is
purely opt-in.

Testing

• tests/test_helpers.py covers the enabled path, the default (shared
  client) path, and the Azure branch, asserting the correct http client is
  passed to the SDK constructor.
• ruff check / ruff format --check clean.
• pytest tests/ passes.

Note: mypy reports one pre-existing error in helpers.py
(Template with a nullable hass argument) that is unrelated to this
change and present on develop.

How to use

Enable Settings → Devices & Services → Extended OpenAI Conversation →
Configure → Follow redirects, or set it at runtime via the
change_config service.