acp: Support specifying settings for extensions (#43177)

This allows you to specify default_model and default_mode for ACP
extensions, e.g.
```
"auggie": {
  "default_model": "gpt-5",
  "default_mode": "default",
  "type": "extension"
},
```

Release Notes:

- Added support for specifying settings for ACP extensions
(`default_mode`, `default_model`)

crates/migrator/src/migrations.rs

@@ -141,3 +141,9 @@ pub(crate) mod m_2025_11_12 {
 
     pub(crate) use settings::SETTINGS_PATTERNS;
 }
+
+pub(crate) mod m_2025_11_20 {
+    mod settings;
+
+    pub(crate) use settings::SETTINGS_PATTERNS;
+}

crates/migrator/src/migrations/m_2025_11_20/settings.rs

@@ -0,0 +1,76 @@
+use std::ops::Range;
+
+use tree_sitter::{Query, QueryMatch};
+
+use crate::MigrationPatterns;
+
+pub const SETTINGS_PATTERNS: MigrationPatterns = &[(
+    SETTINGS_AGENT_SERVERS_CUSTOM_PATTERN,
+    migrate_custom_agent_settings,
+)];
+
+const SETTINGS_AGENT_SERVERS_CUSTOM_PATTERN: &str = r#"(document
+    (object
+        (pair
+            key: (string (string_content) @agent-servers)
+            value: (object
+                (pair
+                    key: (string (string_content) @server-name)
+                    value: (object) @server-settings
+                )
+            )
+        )
+    )
+    (#eq? @agent-servers "agent_servers")
+)"#;
+
+fn migrate_custom_agent_settings(
+    contents: &str,
+    mat: &QueryMatch,
+    query: &Query,
+) -> Option<(Range<usize>, String)> {
+    let server_name_index = query.capture_index_for_name("server-name")?;
+    let server_name = mat.nodes_for_capture_index(server_name_index).next()?;
+    let server_name_text = &contents[server_name.byte_range()];
+
+    if matches!(server_name_text, "gemini" | "claude" | "codex") {
+        return None;
+    }
+
+    let server_settings_index = query.capture_index_for_name("server-settings")?;
+    let server_settings = mat.nodes_for_capture_index(server_settings_index).next()?;
+
+    let mut column = None;
+
+    // Parse the server settings to check what keys it contains
+    let mut cursor = server_settings.walk();
+    for child in server_settings.children(&mut cursor) {
+        if child.kind() == "pair" {
+            if let Some(key_node) = child.child_by_field_name("key") {
+                if let (None, Some(quote_content)) = (column, key_node.child(0)) {
+                    column = Some(quote_content.start_position().column);
+                }
+                if let Some(string_content) = key_node.child(1) {
+                    let key = &contents[string_content.byte_range()];
+                    match key {
+                        // If it already has a type key, don't modify it
+                        "type" => return None,
+                        _ => {}
+                    }
+                }
+            }
+        }
+    }
+
+    // Insert the type key at the beginning of the object
+    let start = server_settings.start_byte() + 1;
+    let indent = " ".repeat(column.unwrap_or(12));
+
+    Some((
+        start..start,
+        format!(
+            r#"
+{indent}"type": "custom","#
+        ),
+    ))
+}

crates/migrator/src/migrator.rs

@@ -219,6 +219,10 @@ pub fn migrate_settings(text: &str) -> Result<Option<String>> {
             migrations::m_2025_11_12::SETTINGS_PATTERNS,
             &SETTINGS_QUERY_2025_11_12,
         ),
+        MigrationType::TreeSitter(
+            migrations::m_2025_11_20::SETTINGS_PATTERNS,
+            &SETTINGS_QUERY_2025_11_20,
+        ),
     ];
     run_migrations(text, migrations)
 }
@@ -341,6 +345,10 @@ define_query!(
     SETTINGS_QUERY_2025_11_12,
     migrations::m_2025_11_12::SETTINGS_PATTERNS
 );
+define_query!(
+    SETTINGS_QUERY_2025_11_20,
+    migrations::m_2025_11_20::SETTINGS_PATTERNS
+);
 
 // custom query
 static EDIT_PREDICTION_SETTINGS_MIGRATION_QUERY: LazyLock<Query> = LazyLock::new(|| {
@@ -1192,6 +1200,63 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_custom_agent_server_settings_migration() {
+        assert_migrate_settings_with_migrations(
+            &[MigrationType::TreeSitter(
+                migrations::m_2025_11_20::SETTINGS_PATTERNS,
+                &SETTINGS_QUERY_2025_11_20,
+            )],
+            r#"{
+    "agent_servers": {
+        "gemini": {
+            "default_model": "gemini-1.5-pro"
+        },
+        "claude": {},
+        "codex": {},
+        "my-custom-agent": {
+            "command": "/path/to/agent",
+            "args": ["--foo"],
+            "default_model": "my-model"
+        },
+        "already-migrated-agent": {
+            "type": "custom",
+            "command": "/path/to/agent"
+        },
+        "future-extension-agent": {
+            "type": "extension",
+            "default_model": "ext-model"
+        }
+    }
+}"#,
+            Some(
+                r#"{
+    "agent_servers": {
+        "gemini": {
+            "default_model": "gemini-1.5-pro"
+        },
+        "claude": {},
+        "codex": {},
+        "my-custom-agent": {
+            "type": "custom",
+            "command": "/path/to/agent",
+            "args": ["--foo"],
+            "default_model": "my-model"
+        },
+        "already-migrated-agent": {
+            "type": "custom",
+            "command": "/path/to/agent"
+        },
+        "future-extension-agent": {
+            "type": "extension",
+            "default_model": "ext-model"
+        }
+    }
+}"#,
+            ),
+        );
+    }
+
     #[test]
     fn test_remove_version_fields() {
         assert_migrate_settings(