#3387 [Engine] reject duplicate GPUs in automatic replica groups

Description

## Problem and result
On the current upstream `main`, `gpu_groups()` accepts overlapping GPUs when a numeric replica count derives groups from the configured GPU list. Reproduced before the fix: `{"gpu":[0,0],"replicas":2}` returns `[[0],[0]]`; `{"gpu":[0,1,1,2],"replicas":2}` returns `[[0,1],[1,2]]`. The server can then start independent replica engines on the same card. After this change, these configurations raise `ReplicaConfigError` before replica engines start.

## Related discussion
[PR #2953](https://github.com/Niko1221/Strata/pull/2953) was closed during the 0.2.0-beta tracker reset, not declined. The original change was revalidated against current `main`. Search of open and closed issues and PRs for `gpu_groups`, duplicate cards and replica GPU grouping found no other fix for this case.

## Approach
Build numeric groups and pass them through the same non-empty and duplicate-card validation used for explicitly named groups. Add regression coverage for duplicate GPU IDs in list and comma-separated forms, normalized duplicate IDs, explicit groups, preserved card order, and disabled replica counts. Valid group ordering and the no-replica path remain unchanged. No engine backend or defaults are changed.

## Validation
**Passed:**
- Reproduced the issue on upstream `main` at `e88ee39d1d6be6e3364183af3b39b62a7ed6f211`, Linux Debian 13, Python 3.13.5: duplicate GPU configurations returned overlapping groups instead of raising an error.
- `uv run --with-requirements requirements.txt python -m unittest serve.test_replicas serve.test_parallel -q`, on the change based on `e88ee39d1d6be6e3364183af3b39b62a7ed6f211`, Linux Debian 13, Python 3.13.5: **64 tests passed** in 71.163 seconds. These tests use fake engines; no model or GPU was used.
- `git diff --check`: passed.

**Failed:**
- The initial `python3 -m unittest serve.test_replicas serve.test_parallel -q` attempt could not import `jinja2` from the system Python. The same suite then passed when run with the pinned repository requirements in a temporary `uv` environment.

**Not run:**
- Multi-GPU inference and CUDA, HIP, SYCL or Metal builds; this change touches only Python server code and was verified with fake engines.

## Limits
This validates configuration handling and fake-engine server behavior. It does not establish GPU runtime behavior or performance on hardware.