exploration: create Session as part of Codex::spawn() (#2291)

Historically, `Codex::spawn()` would create the instance of `Codex` and
enforce, by construction, that `Op::ConfigureSession` was the first `Op`
submitted via `submit()`. Then over in `submission_loop()`, it would
handle the case for taking the parameters of `Op::ConfigureSession` and
turning it into a `Session`.

This approach has two challenges from a state management perspective:


f968a1327a/codex-rs/core/src/codex.rs (L718)

- The local `sess` variable in `submission_loop()` has to be `mut` and
`Option<Arc<Session>>` because it is not invariant that a `Session` is
present for the lifetime of the loop, so there is a lot of logic to deal
with the case where `sess` is `None` (e.g., the `send_no_session_event`
function and all of its callsites).
- `submission_loop()` is written in such a way that
`Op::ConfigureSession` could be observed multiple times, but in
practice, it is only observed exactly once at the start of the loop.

In this PR, we try to simplify the state management by _removing_ the
`Op::ConfigureSession` enum variant and constructing the `Session` as
part of `Codex::spawn()` so that it can be passed to `submission_loop()`
as `Arc<Session>`. The original logic from the `Op::ConfigureSession`
has largely been moved to the new `Session::new()` constructor.

---

Incidentally, I also noticed that the handling of `Op::ConfigureSession`
can result in events being dispatched in addition to
`EventMsg::SessionConfigured`, as an `EventMsg::Error` is created for
every MCP initialization error, so it is important to preserve that
behavior:


f968a1327a/codex-rs/core/src/codex.rs (L901-L916)

Though admittedly, I believe this does not play nice with #2264, as
these error messages will likely be dispatched before the client has a
chance to call `addConversationListener`, so we likely need to make it
so `newConversation` automatically creates the subscription, but we must
also guarantee that the "ack" from `newConversation` is returned before
any other conversation-related notifications are sent so the client
knows what `conversation_id` to match on.

codex-rs/core/src/conversation_manager.rs

@@ -7,6 +7,7 @@ use uuid::Uuid;
 
 use crate::codex::Codex;
 use crate::codex::CodexSpawnOk;
+use crate::codex::INITIAL_SUBMIT_ID;
 use crate::codex_conversation::CodexConversation;
 use crate::config::Config;
 use crate::error::CodexErr;
@@ -52,7 +53,6 @@ impl ConversationManager {
     ) -> CodexResult<NewConversation> {
         let CodexSpawnOk {
             codex,
-            init_id,
             session_id: conversation_id,
         } = Codex::spawn(config, auth).await?;
 
@@ -64,7 +64,7 @@ impl ConversationManager {
             Event {
                 id,
                 msg: EventMsg::SessionConfigured(session_configured),
-            } if id == init_id => session_configured,
+            } if id == INITIAL_SUBMIT_ID => session_configured,
             _ => {
                 return Err(CodexErr::SessionConfiguredNotFirstEvent);
             }