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/codex_message_processor.rs

@@ -1829,7 +1829,7 @@ impl CodexMessageProcessor {
             return;
         }
 
-        let cwd = cwd.unwrap_or_else(|| self.config.cwd.to_path_buf());
+        let cwd = cwd.map_or_else(|| self.config.cwd.clone(), |cwd| self.config.cwd.join(cwd));
         let mut env = create_env(
             &self.config.permissions.shell_environment_policy,
             /*thread_id*/ None,