fix: make all fields of Session private (#2285)

As `Session` needs a bit of work, it will make things easier to move
around if we can start by reducing the extent of its public API. This
makes all the fields private, though adds three `pub(crate)` getters.

---
[//]: # (BEGIN SAPLING FOOTER)
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* #2287
* #2286
* __->__ #2285

codex-rs/core/src/protocol.rs

@@ -723,7 +723,7 @@ pub enum ReviewDecision {
     Abort,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize)]
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum FileChange {
     Add {