Fix remote turn diff display roots (#23261)

## Why

`TurnDiffTracker` computes a display root so turn diffs can be rendered
repo-relative. For remote exec-server turns, the selected turn `cwd` may
exist only inside the selected environment, but `run_turn` was
discovering the git root through the local host filesystem. When that
lookup failed, nested remote-session diffs fell back to the nested `cwd`
and showed `/tmp/...`-prefixed paths instead of repo-relative paths.

## What changed

- Resolve the diff display root from the primary selected turn
environment when one exists, using that environment's filesystem and
`cwd`.
- Add `codex_git_utils::get_git_repo_root_with_fs(...)` so git-root
discovery can run against an `ExecutorFileSystem`, including remote
environments.
- Reuse that helper from `resolve_root_git_project_for_trust(...)` and
add coverage for `.git` gitdir-pointer detection.

## Validation

- Devbox Bazel: `//codex-rs/core:core-unit-tests
--test_filter=get_git_repo_root_with_fs_detects_gitdir_pointer`
- Devbox Docker-backed remote-env repro: `//codex-rs/core:core-all-test
--test_filter=apply_patch_turn_diff_paths_stay_repo_relative_when_session_cwd_is_nested`

codex-rs/git-utils/src/lib.rs

@@ -31,6 +31,7 @@ pub use info::default_branch_name;
 pub use info::get_git_remote_urls;
 pub use info::get_git_remote_urls_assume_git_repo;
 pub use info::get_git_repo_root;
+pub use info::get_git_repo_root_with_fs;
 pub use info::get_has_changes;
 pub use info::get_head_commit_hash;
 pub use info::git_diff_to_remote;