Summary

The GooseFS ObjectStore provider used to bake the URL path into OpenDAL's
root and always return an empty extract_path. Because ObjectStoreRegistry
keys its cache on the provider prefix (goosefs$host:port), two different
dataset URLs under the same master resolved to the same cached
ObjectStore — with a root locked to whichever URL opened it first. The
second open reused that store and, since extract_path was empty, the read
went to the first dataset's bytes and schema.

This PR aligns the GooseFS provider with the S3/GCS/OSS model.

Reproducer (before this fix)

COPY (SELECT range::BIGINT AS col_a FROM range(3))
  TO 'goosefs://host:9200/repro/a.lance' (FORMAT lance);

COPY (SELECT range::BIGINT AS col_b1,
             ('row_' || range)::VARCHAR AS col_b2 FROM range(5))
  TO 'goosefs://host:9200/repro/b.lance' (FORMAT lance);

SELECT * FROM 'goosefs://host:9200/repro/a.lance';   -- OK: 3 rows, col_a
SELECT * FROM 'goosefs://host:9200/repro/b.lance';
-- BUG: returns 3 rows of col_a (a.lance's data & schema), instead of
--      5 rows of (col_b1, col_b2).

Root cause

rust/lance-io/src/object_store/providers/goosefs.rs had three coupled
choices that together broke the registry cache invariants:

1. new_store set root = base_path.path() — a per-URL root.
2. extract_path unconditionally returned Path::from(""), "trusting"
   the root to fully identify the object.
3. calculate_object_store_prefix returned only goosefs$host:port,
   so the URL path did not participate in the cache key.

ObjectStoreRegistry::get_store (rust/lance-io/src/object_store/providers.rs)
keys active_stores on (prefix, params) and holds Weak<ObjectStore>.
As long as any live Arc<ObjectStore> for a.lance exists, opening
b.lance is a cache hit that returns the store rooted at a.lance,
and the empty key from extract_path reads the wrong dataset.

Fix (S3-style path model)

• root is cluster-wide, not per-URL. Defaults to /, overridable
  via storage option goosefs_root or env var GOOSEFS_ROOT. All
  datasets under the same master share a single OpenDAL Operator.
• extract_path returns the URL path (percent-decoded via
  Path::from_url_path) as the per-request object key. OpenDAL's
  build_rooted_abs_path(root, key) reconstructs the absolute GooseFS
  path exactly as before.
• calculate_object_store_prefix stays scoped to goosefs$host:port
  in the default case, and folds a custom root in as goosefs$host:port#/root
  so different roots remain isolated in the cache.

Net effect: a.lance and b.lance share one cached Operator (the
same reuse benefit S3 already gets), and correctness comes from the
distinct object keys on each request.

Semantic parity with other providers

goosefs://host:9200/repro/a.lance
├── prefix        "goosefs$host:9200"        ← cache key (shared)
├── operator root "/"                        ← stable across datasets
└── extract_path  "repro/a.lance"            ← per-request key

This mirrors aws.rs / gcp.rs / oss.rs, which use the default trait
implementations of extract_path and calculate_object_store_prefix.

Tests

Added focused regression tests in the provider (all passing):

• test_prefix_shared_across_datasets_same_master — same master, two
  dataset URLs share one cache prefix but yield distinct object
  keys. This is the direct counter-assertion to the bug.
• test_prefix_isolated_across_masters — different masters must not
  share a cache entry.
• test_prefix_includes_custom_root — a custom goosefs_root is
  reflected in the prefix.
• test_goosefs_extract_path_percent_decoded — percent-encoded segments
  (e.g. %20) are decoded exactly once, guarding against double-encoding
  when the object store client re-encodes for transport.

Verification

• cargo fmt -p lance-io -- --check → clean
• cargo clippy -p lance-io --features goosefs --all-targets -- -D warnings → 0 warnings
• cargo test -p lance-io --features goosefs --lib → 197 passed / 0 failed

Compatibility

• URL scheme, storage options, and the on-wire GooseFS paths are unchanged.
• Users who previously relied on the (undocumented) side effect of the
  URL path becoming the OpenDAL root need no change — the same absolute
  path is still resolved via root="/" + relative key.
• New optional knob: goosefs_root / GOOSEFS_ROOT for tenants that want
  to sandbox an Operator to a subtree. Different values get isolated
  cache entries.