acp: Automatically install gemini under Zed's data dir (#37054)

Closes: https://github.com/zed-industries/zed/issues/37089

Instead of looking for the gemini command on `$PATH`, by default we'll
install our own copy on demand under our data dir, as we already do for
language servers and debug adapters. This also means we can handle
keeping the binary up to date instead of prompting the user to upgrade.

Notes:

- The download is only triggered if you open a new Gemini thread
- Custom commands from `agent_servers.gemini` in settings are respected
as before
- A new `agent_servers.gemini.ignore_system_version` setting is added,
similar to the existing settings for language servers. It's `true` by
default, and setting it to `false` disables the automatic download and
makes Zed search `$PATH` as before.
- If `agent_servers.gemini.ignore_system_version` is `false` and no
binary is found on `$PATH`, we'll fall back to automatic installation.
If it's `false` and a binary is found, but the version is older than
v0.2.1, we'll show an error.

Release Notes:

- acp: By default, Zed will now download and use a private copy of the
Gemini CLI binary, instead of searching your `$PATH`. To make Zed search
your `$PATH` for Gemini CLI before attempting to download it, use the
following setting:

```
{
  "agent_servers": {
    "gemini": {
      "ignore_system_version": false
    }
  }
}
```

crates/agent2/src/native_agent_server.rs

@@ -1,10 +1,9 @@
 use std::{any::Any, path::Path, rc::Rc, sync::Arc};
 
-use agent_servers::AgentServer;
+use agent_servers::{AgentServer, AgentServerDelegate};
 use anyhow::Result;
 use fs::Fs;
 use gpui::{App, Entity, SharedString, Task};
-use project::Project;
 use prompt_store::PromptStore;
 
 use crate::{HistoryStore, NativeAgent, NativeAgentConnection, templates::Templates};
@@ -30,33 +29,21 @@ impl AgentServer for NativeAgentServer {
         "Zed Agent".into()
     }
 
-    fn empty_state_headline(&self) -> SharedString {
-        self.name()
-    }
-
-    fn empty_state_message(&self) -> SharedString {
-        "".into()
-    }
-
     fn logo(&self) -> ui::IconName {
         ui::IconName::ZedAgent
     }
 
-    fn install_command(&self) -> Option<&'static str> {
-        None
-    }
-
     fn connect(
         &self,
         _root_dir: &Path,
-        project: &Entity<Project>,
+        delegate: AgentServerDelegate,
         cx: &mut App,
     ) -> Task<Result<Rc<dyn acp_thread::AgentConnection>>> {
         log::debug!(
             "NativeAgentServer::connect called for path: {:?}",
             _root_dir
         );
-        let project = project.clone();
+        let project = delegate.project().clone();
         let fs = self.fs.clone();
         let history = self.history.clone();
         let prompt_store = PromptStore::global(cx);