chore: upgrade to Rust 1.92.0 (#8860)

**Summary**
- Upgrade Rust toolchain used by CI to 1.92.0.
- Address new clippy `derivable_impls` warnings by deriving `Default`
for enums across protocol, core, backend openapi models, and
windows-sandbox setup.
- Tidy up related test/config behavior (originator header handling, env
override cleanup) and remove a now-unused assignment in TUI/TUI2 render
layout.

**Testing**
- `just fmt`
- `just fix -p codex-tui`
- `just fix -p codex-tui2`
- `just fix -p codex-windows-sandbox`
- `cargo test -p codex-tui`
- `cargo test -p codex-tui2`
- `cargo test -p codex-windows-sandbox`
- `cargo test -p codex-core --test all`
- `cargo test -p codex-app-server --test all`
- `cargo test -p codex-mcp-server --test all`
- `cargo test --all-features`

codex-rs/protocol/src/protocol.rs

@@ -1656,21 +1656,18 @@ pub struct ReviewLineRange {
     pub end: u32,
 }
 
-#[derive(Debug, Clone, Copy, Display, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[derive(
+    Debug, Clone, Copy, Display, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS, Default,
+)]
 #[serde(rename_all = "snake_case")]
 pub enum ExecCommandSource {
+    #[default]
     Agent,
     UserShell,
     UnifiedExecStartup,
     UnifiedExecInteraction,
 }
 
-impl Default for ExecCommandSource {
-    fn default() -> Self {
-        Self::Agent
-    }
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct ExecCommandBeginEvent {
     /// Identifier so this can be paired with the ExecCommandEnd event.