Language model transport errors currently retain only a provider display
name after an HTTP request fails. Downstream clients therefore cannot
tell users which configured endpoint was unreachable without parsing the
source error or hard-coding provider names.

Make the request hostname a required part of
`LanguageModelCompletionError::HttpSend` and preserve it through the
Anthropic, OpenAI-compatible, OpenRouter, Vercel AI Gateway, and Zed
Cloud transports. Each transport captures the parsed hostname from the
request URI before sending it, using the configured base URL as the
defensive fallback. This supports production, development, and custom
endpoints without string matching.

Tests exercise transport failures through the real request paths and
verify hostnames for Anthropic, ChatGPT Subscription, generic
OpenAI-compatible providers, and Zed Cloud.

Testing:

- `cargo fmt --check`
- `cargo nextest run -p anthropic -p open_ai -p open_router -p
language_models_cloud -p language_models` — 209 tests passed
- `cargo check -p agent -p agent_ui --all-targets`
- `git diff --check`
- `./script/clippy -p language_model_core -p anthropic -p open_ai -p
open_router -p language_models_cloud -p language_models` — clippy
passed; the script subsequently failed in the repository-wide `cargo
shear --deny-warnings` step on 203 pre-existing target and doctest
configuration warnings

Release Notes:

- Improved language model connection errors to identify the unreachable
host.