fix: ensure accept_elicitation_for_prompt_rule() test passes locally (#7832)

When I originally introduced `accept_elicitation_for_prompt_rule()` in
https://github.com/openai/codex/pull/7617, it worked for me locally
because I had run `codex-rs/exec-server/tests/suite/bash` once myself,
which had the side-effect of installing the corresponding DotSlash
artifact.

In CI, I added explicit logic to do this as part of
`.github/workflows/rust-ci.yml`, which meant the test also passed in CI,
but this logic should have been done as part of the test so that it
would work locally for devs who had not installed the DotSlash artifact
for `codex-rs/exec-server/tests/suite/bash` before. This PR updates the
test to do this (and deletes the setup logic from `rust-ci.yml`),
creating a new `DOTSLASH_CACHE` in a temp directory so that this is
handled independently for each test.

While here, also added a check to ensure that the `codex` binary has
been built prior to running the test, as we have to ensure it is
symlinked as `codex-linux-sandbox` on Linux in order for the integration
test to work on that platform.

.github/workflows/rust-ci.yml

@@ -412,14 +412,6 @@ jobs:
       - name: Install DotSlash
         uses: facebook/install-dotslash@v2
 
-      - name: Pre-fetch DotSlash artifacts
-        # The Bash wrapper is not available on Windows.
-        if: ${{ !startsWith(matrix.runner, 'windows') }}
-        shell: bash
-        run: |
-          set -euo pipefail
-          dotslash -- fetch exec-server/tests/suite/bash
-
       - uses: dtolnay/rust-toolchain@1.90
         with:
           targets: ${{ matrix.target }}