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fix(darker): fail the linter job when darker crashes DEV-1034 (#7316)

### 📣 Summary

No user-facing change — this fixes an internal code-quality check that
was reporting success even when it had not actually run.

### 📖 Description

The automated Python style check that runs on every code change could
fail to start and still report a green result, so problems it was meant
to catch could slip through unnoticed. It now reports a clear failure
whenever it cannot complete.

### 💭 Notes

The step captured darker's stdout and failed only when that string was
non-empty. Darker writes crash details to **stderr**, so every crash
produced empty stdout and took the `|| exit 0` branch. `shell:
/usr/bin/bash {0}` also dropped GitHub's default `-e -o pipefail`, so
nothing else could abort the step either.

- Trust darker's exit code instead of sniffing stdout — measured on the
pinned `darker[isort]==2.1.1`: clean run exits **0**, findings and
crashes exit non-zero.
- The comment justifying the workaround ("darker still exits with code 1
even with no errors") is stale and no longer true; removed so it doesn't
invite a re-fix.
- `shell: bash` (keyword form) restores `-e -o pipefail`.
- Added an explicit guard for an unresolvable base SHA: `git rev-list`
*succeeds* on a parentless commit, so `set -e` alone wouldn't catch it.

Verified locally against a scratch repo under `bash --noprofile --norc
-eo pipefail`:

| Scenario | rc | before | after |
|---|---|---|---|
| clean tree | 0 | ✅ pass | ✅ pass |
| bad formatting / flake8 finding | 1 | ❌ fail | ❌ fail |
| unknown revision | 128 | ✅ **silent pass** | ❌ fail |
| flake8 binary missing | 1 | ✅ **silent pass** | ❌ fail |
| darker binary missing | 127 | ✅ **silent pass** | ❌ fail |
| bad darker flag | 2 | ✅ **silent pass** | ❌ fail |
| base SHA unresolvable | — | ✅ **silent pass** | ❌ fail + `::error::` |

Follow-up to #6263, which pinned darker but left the silent-pass wrapper
in place. `darker.yml` was the only workfl... (continued)

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