[CI] Fix shallow vLLM range recovery (#15306)

### What this PR does / why we need it?

This follow-up fixes the shallow-checkout range resolution failure
observed after #14560 was merged.

In [vLLM Buildkite build
#86104](https://buildkite.com/vllm/ci/builds/86104/list?sid=01a04bab-e537-447f-b487-3d9f8eca4f1f&tab=output),
the new interface test was invoked successfully but stopped before
source indexing:

```text
Git command failed with exit code 1:
git -C /workspace/vllm merge-base d4d035ccef298dda06e0f490234b76381f8b859f FETCH_HEAD
The Git command produced no output.
```

The detailed Git diagnostics added before #14560 was merged converted
`subprocess.CalledProcessError` into `RuntimeError`. However,
`resolve_vllm_range()` still caught `subprocess.CalledProcessError`, so
the existing shallow-repository recovery path could no longer run.

This PR introduces a narrow `GitCommandError` type and uses it
consistently:

```text
_git() failure
  -> raise GitCommandError with command/stdout/stderr
  -> resolve_vllm_range() catches GitCommandError
  -> verify the checkout is shallow
  -> fetch --unshallow
  -> retry merge-base
```

The change retains the actionable pytest diagnostics while restoring the
intended `--unshallow` retry. Other `RuntimeError` instances are not
caught.

### Does this PR introduce _any_ user-facing change?

No. Runtime inference behavior and public APIs are unchanged. This only
fixes vLLM PR CI range resolution.

### How was this patch tested?

A local bare Git remote was used to create this history:

```text
        B  main tip
       /
A  merge base
       \
        P  PR head
```

Two independent `--depth 1` clones of the PR branch were created. Both
implementations executed real Git commands; the harness only recorded
the command arguments.

| Result | Before this fix | After this fix |
| --- | --- | --- |
| Initial shallow state | `true` | `true` |
| Initial `merge-base` | failed | failed |
| `fetch --unshallow` observed | no | yes |
| Range resolution | failed with the Buildkite error | passed |
| Final shallow state | `true` | `false` |
| Base/head SHAs | not returned | both matched exactly |

Additional checks:

```bash
python -m ruff check tests/e2e/vllm_interface/vllm_interface_contracts/vllm_ci.py
python -m ruff format --check tests/e2e/vllm_interface/vllm_interface_contracts/vllm_ci.py
python -m mypy --follow-imports skip --check-untyped-defs --python-version 3.10 tests/e2e/vllm_interface/vllm_interface_contracts/vllm_ci.py
python -m mypy --follow-imports skip --check-untyped-defs --python-version 3.11 tests/e2e/vllm_interface/vllm_interface_contracts/vllm_ci.py
python -m mypy --follow-imports skip --check-untyped-defs --python-version 3.12 tests/e2e/vllm_interface/vllm_interface_contracts/vllm_ci.py
python -m compileall -q tests/e2e/vllm_interface/vllm_interface_contracts/vllm_ci.py
git diff --check origin/main..HEAD
```

All checks passed.
- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

Signed-off-by: shenzhao <shenzhao9@huawei.com>
Co-authored-by: shenzhao <shenzhao9@huawei.com>