Use AbsolutePathBuf for exec cwd plumbing (#17063)

## Summary
- Carry `AbsolutePathBuf` through tool cwd parsing/resolution instead of
resolving workdirs to raw `PathBuf`s.
- Type exec/sandbox request cwd fields as `AbsolutePathBuf` through
`ExecParams`, `ExecRequest`, `SandboxCommand`, and unified exec runtime
requests.
- Keep `PathBuf` conversions at external/event boundaries and update
existing tests/fixtures for the typed cwd.

## Validation
- `cargo check -p codex-core --tests`
- `cargo check -p codex-sandboxing --tests`
- `cargo test -p codex-sandboxing`
- `cargo test -p codex-core --lib tools::handlers::`
- `just fix -p codex-sandboxing`
- `just fix -p codex-core`
- `just fmt`

Full `codex-core` test suite was not run locally; per repo guidance I
kept local validation targeted.

codex-rs/app-server/src/command_exec.rs

@@ -708,13 +708,13 @@ fn internal_error(message: String) -> JSONRPCErrorError {
 #[cfg(test)]
 mod tests {
     use std::collections::HashMap;
-    use std::path::PathBuf;
 
     use codex_protocol::config_types::WindowsSandboxLevel;
     use codex_protocol::permissions::FileSystemSandboxPolicy;
     use codex_protocol::permissions::NetworkSandboxPolicy;
     use codex_protocol::protocol::ReadOnlyAccess;
     use codex_protocol::protocol::SandboxPolicy;
+    use codex_utils_absolute_path::AbsolutePathBuf;
     use pretty_assertions::assert_eq;
     #[cfg(not(target_os = "windows"))]
     use tokio::time::Duration;
@@ -736,7 +736,7 @@ mod tests {
         };
         ExecRequest::new(
             vec!["cmd".to_string()],
-            PathBuf::from("."),
+            AbsolutePathBuf::current_dir().expect("current dir"),
             HashMap::new(),
             /*network*/ None,
             ExecExpiration::DefaultTimeout,
@@ -848,7 +848,7 @@ mod tests {
                 process_id: Some("proc-100".to_string()),
                 exec_request: ExecRequest::new(
                     vec!["sh".to_string(), "-lc".to_string(), "sleep 30".to_string()],
-                    PathBuf::from("."),
+                    AbsolutePathBuf::current_dir().expect("current dir"),
                     HashMap::new(),
                     /*network*/ None,
                     ExecExpiration::Cancellation(CancellationToken::new()),