Allow customization of the model used for tool calling (#15479)

We also eliminate the `completion` crate and moved its logic into
`LanguageModelRegistry`.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/assistant/Cargo.toml

@@ -32,7 +32,6 @@ client.workspace = true
 clock.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
-completion.workspace = true
 editor.workspace = true
 fs.workspace = true
 futures.workspace = true
@@ -77,7 +76,6 @@ workspace.workspace = true
 picker.workspace = true
 
 [dev-dependencies]
-completion = { workspace = true, features = ["test-support"] }
 ctor.workspace = true
 editor = { workspace = true, features = ["test-support"] }
 env_logger.workspace = true