language_models: Dynamically detect Copilot Chat models (#29027)

I noticed the discussion in #28881, and had thought of exactly the same
a few days prior.

This implementation should preserve existing functionality fairly well.

I've added a dependency (serde_with) to allow the deserializer to skip
models which cannot be deserialized, which could occur if a future
provider, for instance, is added. Without this modification, such a
change could break all models. If extra dependencies aren't desired, a
manual implementation could be used instead.

- Closes #29369 

Release Notes:

- Dynamically detect available Copilot Chat models, including all models
with tool support

---------

Co-authored-by: AidanV <aidanvanduyne@gmail.com>
Co-authored-by: imumesh18 <umesh4257@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Agus Zubiaga <hi@aguz.me>

Cargo.lock

@@ -3309,6 +3309,7 @@ dependencies = [
  "http_client",
  "indoc",
  "inline_completion",
+ "itertools 0.14.0",
  "language",
  "log",
  "lsp",
@@ -3318,11 +3319,9 @@ dependencies = [
  "paths",
  "project",
  "rpc",
- "schemars",
  "serde",
  "serde_json",
  "settings",
- "strum 0.27.1",
  "task",
  "theme",
  "ui",