fix: preserve namespace read storage options

[LuciferYang]

Summary

• fix namespace-backed reads to request vended storage credentials from describe_table()
• merge namespace storage options with user-provided storage_options, with namespace values taking precedence
• reopen the resolved table URI on Ray workers with the merged storage options
• preserve namespace table id, integer table version, and namespace-managed versioning when reconstructing worker datasets
• add regression tests for namespace reads, direct MinIO-style URI reads, and worker reconstruction
• update namespace docs/examples to use namespace_impl / namespace_properties and call out the MinIO S3 API endpoint

Semantics

For namespace reads, the driver resolves the table once through the namespace before creating Ray read tasks. The resolved URI and merged storage options are then captured in each read task so workers can open the same table location without losing object-store credentials.

If a namespace returns managed_versioning=True, workers pass namespace_client_managed_versioning=True when reopening the dataset. Integer dataset_options["version"] values are included in the namespace describe request; non-integer versions are still passed to Lance when opening the dataset.

The dependency lower bound is set to lance-namespace>=0.7.6, which is the locked version that already includes vend_credentials and managed_versioning support.

Validation

• python -m pytest tests/test_datasource_namespace_options.py tests/test_basic_read_write.py::TestNamespaceReadWrite -q
• python -m ruff check lance_ray/datasource.py tests/test_datasource_namespace_options.py
• python -m ruff format --check lance_ray/datasource.py tests/test_datasource_namespace_options.py
• local MinIO smoke test with namespace write/read using endpoint=http://localhost:9000

Fixes #70

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[LuciferYang]

Verified the #70 path with a local MinIO smoke test: namespace-backed write/read using the S3 API endpoint http://localhost:9000 and Lance storage_options for credentials. The read path returned the expected rows through Ray workers.