Extract codex-plugin crate (#15747)

## Summary
- extract plugin identifiers and load-outcome types into codex-plugin
- update codex-core to consume the new plugin crate

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/plugin/src/plugin_id.rs

@@ -0,0 +1,64 @@
+//! Stable plugin identifier parsing and validation shared with the plugin cache.
+
+#[derive(Debug, thiserror::Error)]
+pub enum PluginIdError {
+    #[error("{0}")]
+    Invalid(String),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PluginId {
+    pub plugin_name: String,
+    pub marketplace_name: String,
+}
+
+impl PluginId {
+    pub fn new(plugin_name: String, marketplace_name: String) -> Result<Self, PluginIdError> {
+        validate_plugin_segment(&plugin_name, "plugin name").map_err(PluginIdError::Invalid)?;
+        validate_plugin_segment(&marketplace_name, "marketplace name")
+            .map_err(PluginIdError::Invalid)?;
+        Ok(Self {
+            plugin_name,
+            marketplace_name,
+        })
+    }
+
+    pub fn parse(plugin_key: &str) -> Result<Self, PluginIdError> {
+        let Some((plugin_name, marketplace_name)) = plugin_key.rsplit_once('@') else {
+            return Err(PluginIdError::Invalid(format!(
+                "invalid plugin key `{plugin_key}`; expected <plugin>@<marketplace>"
+            )));
+        };
+        if plugin_name.is_empty() || marketplace_name.is_empty() {
+            return Err(PluginIdError::Invalid(format!(
+                "invalid plugin key `{plugin_key}`; expected <plugin>@<marketplace>"
+            )));
+        }
+
+        Self::new(plugin_name.to_string(), marketplace_name.to_string()).map_err(|err| match err {
+            PluginIdError::Invalid(message) => {
+                PluginIdError::Invalid(format!("{message} in `{plugin_key}`"))
+            }
+        })
+    }
+
+    pub fn as_key(&self) -> String {
+        format!("{}@{}", self.plugin_name, self.marketplace_name)
+    }
+}
+
+/// Validates a single path segment used in plugin IDs and cache layout.
+pub fn validate_plugin_segment(segment: &str, kind: &str) -> Result<(), String> {
+    if segment.is_empty() {
+        return Err(format!("invalid {kind}: must not be empty"));
+    }
+    if !segment
+        .chars()
+        .all(|ch| ch.is_ascii_alphanumeric() || ch == '-' || ch == '_')
+    {
+        return Err(format!(
+            "invalid {kind}: only ASCII letters, digits, `_`, and `-` are allowed"
+        ));
+    }
+    Ok(())
+}