Use ConversationId instead of raw Uuids (#3282)

We're trying to migrate from `session_id: Uuid` to `conversation_id:
ConversationId`. Not only does this give us more type safety but it
unifies our terminology across Codex and with the implementation of
session resuming, a conversation (which can span multiple sessions) is
more appropriate.

I started this impl on https://github.com/openai/codex/pull/3219 as part
of getting resume working in the extension but it's big enough that it
should be broken out.

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -517,7 +517,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             }
             EventMsg::SessionConfigured(session_configured_event) => {
                 let SessionConfiguredEvent {
-                    session_id,
+                    session_id: conversation_id,
                     model,
                     history_log_id: _,
                     history_entry_count: _,
@@ -528,7 +528,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                     self,
                     "{} {}",
                     "codex session".style(self.magenta).style(self.bold),
-                    session_id.to_string().style(self.dimmed)
+                    conversation_id.to_string().style(self.dimmed)
                 );
 
                 ts_println!(self, "model: {}", model);