assistant2: Add live context type and use in message editor (#22865)

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

Cargo.lock

@@ -462,6 +462,7 @@ dependencies = [
  "async-watch",
  "chrono",
  "client",
+ "clock",
  "collections",
  "command_palette_hooks",
  "context_server",
@@ -476,6 +477,7 @@ dependencies = [
  "html_to_markdown",
  "http_client",
  "indoc",
+ "itertools 0.13.0",
  "language",
  "language_model",
  "language_model_selector",