Support opening a single .lance dataset URI

[gordonmurray]

Summary

The opt-in location flow added by #84 accepts a LanceDB database root and then lists the tables beneath it. It does not open a URI that points directly at one .lance dataset.

For example, this works as a database location:

s3://bucket/path
└── table.lance

But entering the direct dataset URI below connects at the wrong level and produces an empty table list:

s3://bucket/path/table.lance

PR #71 supported the direct form through a separate pylance API, but that branch was superseded by the current connection flow.

Proposed approach

Extend the existing opt-in location flow instead of adding another API surface:

• Detect when the supplied location identifies a single .lance dataset.
• Split it into the parent LanceDB location and table name.
• Open it with the existing lancedb.connect(parent) and table-opening code.
• Return the identified table as the single /datasets result.
• Reuse the existing metadata, rows, serialization, error handling, and main/version/tag/branch reference support.
• Do not add a direct pylance dependency or separate /dataset/* endpoints.
• Keep the location field opt-in through the existing DATA_PATH= configuration.

Tests

• Local direct .lance path.
• Object-store URI parsing without rewriting the scheme.
• Database-root behavior remains unchanged.
• Invalid and nonexistent direct locations.
• Full supported LanceDB version matrix.

An object-store integration smoke test against MinIO for the canonical image would be useful if it can remain lightweight.

Related: #12, #71, #84.