Dedupe AssistantSettings (#23190)

This PR dedupes the `AssistantSettings` so we can use the same settings
for both Assistant1 and Assistant2.

We originally forked them so we could change the Assistant2 settings
freely, but given our rollout strategy for the new Assistant, I don't
think that makes sense.

This also fixes the issue where the JSON language server would show a
"Matches multiple schemas when only one must validate" warning in
`settings.json`.

Closes #23171.

Release Notes:

- Fixed the "Matches multiple schemas when only one must validate"
warning for the `assistant` setting.

crates/zed/Cargo.toml

@@ -20,6 +20,7 @@ anyhow.workspace = true
 assets.workspace = true
 assistant.workspace = true
 assistant2.workspace = true
+assistant_settings.workspace = true
 assistant_tools.workspace = true
 async-watch.workspace = true
 audio.workspace = true