fix(data): provide _dataset_uri fallback on DatasetProvider

[genisis0x]

Summary

Fixes #1843. When qlib runs without a `DatasetCache` wrapper, the global `DatasetD` `Wrapper` is registered with a bare `LocalDatasetProvider` instance. `LocalProvider.features_uri` (`qlib/data/data.py:1221`) then calls:

```python
return DatasetD._dataset_uri(instruments, fields, start_time, end_time, freq, disk_cache)
```

That goes through `Wrapper.getattr` to find `_dataset_uri` on the provider — and `LocalDatasetProvider` doesn't have one. The cache-aware override lives on `DatasetCache` / `DiskDatasetCache` only, so the no-cache code path crashes with:

```
AttributeError: 'LocalDatasetProvider' object has no attribute '_dataset_uri'
```

Approach

Add a base `_dataset_uri` to `DatasetProvider` that returns `""` by convention. That mirrors the existing `""` = "no URI, fetch directly" signal that `DiskDatasetCache._dataset_uri` already emits on its `disk_cache == 0` branch (`qlib/data/cache.py:750`), so the contract isn't a new invention — the caller already handles `""` correctly.

Cache subclasses keep their explicit overrides, so the wrapped-with-cache path is unchanged. The fallback also covers `ClientDatasetProvider` and any third-party providers that subclass `DatasetProvider` without routing through a cache, not just `LocalDatasetProvider` specifically.

Test

New `tests/misc/test_dataset_provider_uri.py` with three regressions:

• attribute presence on `LocalDatasetProvider` (the AttributeError pinned exactly)
• the empty-string return for the standard parameter shape
• stability across `disk_cache` values 0/1/2 so a future refactor can't reintroduce the crash by adding a misplaced branch

```
$ .venv/bin/python -m pytest tests/misc/test_dataset_provider_uri.py -v
tests/misc/test_dataset_provider_uri.py::DatasetProviderURITest::test_disk_cache_value_is_ignored_in_fallback PASSED
tests/misc/test_dataset_provider_uri.py::DatasetProviderURITest::test_local_dataset_provider_has_dataset_uri PASSED
tests/misc/test_dataset_provider_uri.py::DatasetProviderURITest::test_local_dataset_provider_returns_empty_uri PASSED
============================== 3 passed in 0.94s ===============================
```

Fixes #1843

[genisis0x]

Read through the CLA — all good. @microsoft-github-policy-service agree