Move test-only deps to dev-dependencies in git_xet (#767)

## Summary
- Move `russh`, `rand_core`, and `tempfile` from regular dependencies to
dev-dependencies in `git_xet`, since they are only used in test code
- `russh` and `rand_core` are also declared as optional regular deps
activated by the `git-xet-for-integration-test` feature flag, since the
integration test SSH server is compiled into the library under that
feature
- Gate `test_utils/ssh_server` module and related exports behind
`#[cfg(any(test, feature = "git-xet-for-integration-test"))]`
- Gate `tests/test_ssh.rs` integration test file behind `#![cfg(feature
= "git-xet-for-integration-test")]`

## Test plan
- [x] `cargo check -p git_xet` passes (no features)
- [x] `cargo test -p git_xet --no-run` passes (no features)
- [x] `cargo test -p git_xet --features git-xet-for-integration-test
--no-run` passes

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: primarily Cargo dependency/feature and `cfg` gating changes,
with no production logic changes; risk is limited to build/test
configuration and feature-flagged integration test coverage.
> 
> **Overview**
> **Reduces default build dependencies for `git_xet`.** Moves `russh`,
`rand_core`, and `tempfile` into `dev-dependencies`, and keeps
`russh`/`rand_core` available as *optional* deps enabled only by the
`git-xet-for-integration-test` feature.
> 
> **Gates SSH test helpers and integration tests behind a feature
flag.** Exposes `GitLFSAuthenticateResponse*` and the local SSH test
server only under `#[cfg(test)]` or `feature =
"git-xet-for-integration-test"`, and makes `tests/test_ssh.rs` compile
only when that feature is enabled.
> 
> Separately, cleans up workspace manifests/lockfiles by moving some
crates (`half`, `regex`, `futures-util`) to dev-deps where they’re only
needed for tests/benches, and adds `.worktrees/` to `.gitignore`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
cdc30a5a8f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

.gitignore

@@ -19,3 +19,4 @@ debug/
 !.vscode/settings.json
 venv
 **/*.env
+.worktrees/