Prepare external agent migration for source adapters (#32884)

## What changed

- Add an optional `source` selector to `externalAgentConfig/detect`, while preserving the existing behavior for omitted, `null`, and unrecognized values.
- Pass `claude-code` as the source for TUI detection and import requests.
- Move source-specific migration behavior behind an adapter boundary and consolidate session migration into `codex-external-agent-migration`.

## Testing

- Cover explicit, omitted, and unknown detection source values with app-server protocol tests.
- Add unit coverage for the extracted migration and hook helpers.

GitOrigin-RevId: 63bd3dcb485f3ad7bd996022db4e351506aeea77

codex-rs/app-server-protocol/src/protocol/v2/config.rs

@@ -684,6 +684,10 @@ pub struct ExternalAgentConfigDetectParams {
     /// Zero or more working directories to include for repo-scoped detection.
     #[ts(optional = nullable)]
     pub cwds: Option<Vec<PathBuf>>,
+    /// Optional import-source selector. Missing or unrecognized values use the default source for
+    /// backwards compatibility.
+    #[ts(optional = nullable)]
+    pub source: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
@@ -691,7 +695,9 @@ pub struct ExternalAgentConfigDetectParams {
 #[ts(export_to = "v2/")]
 pub struct ExternalAgentConfigImportParams {
     pub migration_items: Vec<ExternalAgentConfigMigrationItem>,
-    /// Source product that produced the migration items. Missing means unspecified.
+    /// Import-source selector used to produce the migration items. Pass the same value to detection
+    /// and import; missing or unrecognized values use the default source for backwards
+    /// compatibility.
     #[ts(optional = nullable)]
     pub source: Option<String>,
 }