Move external agent migration into its crate (#33456)

## What changed

- Move migration detection, import orchestration, models, and reporting from
  `codex-app-server` into `codex-external-agent-migration`.
- Keep app-server protocol conversion in a dedicated boundary module.
- Organize source-specific configuration, sessions, hooks, plugins, commands,
  subagents, and memory handling behind shared migration adapters.
- Relocate and split the migration tests alongside the extracted service.

GitOrigin-RevId: 3378ec2064806a28136d6ac98664ca1d75414a4e

codex-rs/external-agent-migration/src/reporting.rs

@@ -0,0 +1,105 @@
+use std::path::Path;
+
+use crate::model::ExternalAgentConfigImportItemResult;
+use crate::model::ExternalAgentConfigImportRawError;
+use crate::model::ExternalAgentConfigMigrationItemType;
+use crate::model::PluginImportOutcome;
+
+fn migration_item_type_label(item_type: ExternalAgentConfigMigrationItemType) -> &'static str {
+    match item_type {
+        ExternalAgentConfigMigrationItemType::Config => "config",
+        ExternalAgentConfigMigrationItemType::Skills => "skills",
+        ExternalAgentConfigMigrationItemType::AgentsMd => "agents_md",
+        ExternalAgentConfigMigrationItemType::Plugins => "plugins",
+        ExternalAgentConfigMigrationItemType::McpServerConfig => "mcp_server_config",
+        ExternalAgentConfigMigrationItemType::Subagents => "subagents",
+        ExternalAgentConfigMigrationItemType::Hooks => "hooks",
+        ExternalAgentConfigMigrationItemType::Commands => "commands",
+        ExternalAgentConfigMigrationItemType::Memory => "memory",
+        ExternalAgentConfigMigrationItemType::Sessions => "sessions",
+    }
+}
+
+pub fn record_import_error(
+    result: &mut ExternalAgentConfigImportItemResult,
+    failure_stage: &'static str,
+    message: impl Into<String>,
+    source: Option<String>,
+) {
+    result.record_error(ExternalAgentConfigImportRawError {
+        item_type: result.item_type,
+        error_type: None,
+        sub_error_type: None,
+        failure_stage: failure_stage.to_string(),
+        message: message.into(),
+        cwd: result.cwd.clone(),
+        source,
+    });
+}
+
+pub(super) fn record_plugin_import_errors(
+    outcome: &mut PluginImportOutcome,
+    cwd: Option<&Path>,
+    plugin_ids: &[String],
+    failure_stage: &'static str,
+    message: impl Into<String>,
+) {
+    let message = message.into();
+    outcome
+        .raw_errors
+        .extend(plugin_ids.iter().map(|plugin_id| {
+            plugin_import_raw_error(cwd, failure_stage, message.clone(), Some(plugin_id.clone()))
+        }));
+}
+
+pub(super) fn plugin_import_raw_error(
+    cwd: Option<&Path>,
+    failure_stage: &'static str,
+    message: String,
+    source: Option<String>,
+) -> ExternalAgentConfigImportRawError {
+    ExternalAgentConfigImportRawError {
+        item_type: ExternalAgentConfigMigrationItemType::Plugins,
+        error_type: None,
+        sub_error_type: None,
+        failure_stage: failure_stage.to_string(),
+        message,
+        cwd: cwd.map(Path::to_path_buf),
+        source,
+    }
+}
+
+pub(super) fn migration_metric_tags(
+    item_type: ExternalAgentConfigMigrationItemType,
+    skills_count: Option<usize>,
+) -> Vec<(&'static str, String)> {
+    let mut tags = vec![(
+        "migration_type",
+        migration_item_type_label(item_type).to_string(),
+    )];
+    if matches!(
+        item_type,
+        ExternalAgentConfigMigrationItemType::Skills
+            | ExternalAgentConfigMigrationItemType::Subagents
+            | ExternalAgentConfigMigrationItemType::Commands
+    ) {
+        tags.push(("skills_count", skills_count.unwrap_or(0).to_string()));
+    }
+    tags
+}
+
+pub(super) fn emit_migration_metric(
+    metric_name: &str,
+    item_type: ExternalAgentConfigMigrationItemType,
+    skills_count: Option<usize>,
+) {
+    let Some(metrics) = codex_otel::global() else {
+        return;
+    };
+    let tags = migration_metric_tags(item_type, skills_count);
+    let tag_refs = tags
+        .iter()
+        .map(|(key, value)| (*key, value.as_str()))
+        .collect::<Vec<_>>();
+    let _ = metrics.counter(metric_name, /*inc*/ 1, &tag_refs);
+}