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/core/src/tools/handlers/read_file.rs

@@ -40,9 +40,10 @@ struct ReadFileArgs {
     indentation: Option<IndentationArgs>,
 }
 
-#[derive(Deserialize)]
+#[derive(Deserialize, Default)]
 #[serde(rename_all = "snake_case")]
 enum ReadMode {
+    #[default]
     Slice,
     Indentation,
 }
@@ -461,12 +462,6 @@ mod defaults {
         }
     }
 
-    impl Default for ReadMode {
-        fn default() -> Self {
-            Self::Slice
-        }
-    }
-
     pub fn offset() -> usize {
         1
     }