feat(storage): add vector buckets support

[spydon]

What

Implements the Storage Vector Buckets subsystem in storage_client, closing all 14 storage.vector_buckets.* gaps in the compliance matrix.

Reachable through supabase.storage.vectors, mirroring the supabase-js StorageVectorsClient structure while staying idiomatic to the Dart storage client (operations return parsed data directly and throw StorageException on failure instead of a {data, error} envelope).

final vectors = supabase.storage.vectors;
await vectors.createBucket('embeddings');

final bucket = vectors.from('embeddings');
await bucket.createIndex(
  name: 'documents',
  dimension: 1536,
  distanceMetric: DistanceMetric.cosine,
);

final index = bucket.index('documents');
await index.putVectors([
  Vector(key: 'doc-1', data: [0.1, 0.2, 0.3], metadata: {'title': 'Intro'}),
]);

final result = await index.queryVectors(queryVector: [0.1, 0.2, 0.3], topK: 5);

Capabilities

Bucket & index management (SupabaseVectorsClient, StorageVectorBucketApi):

• create / list / delete vector bucket, get bucket metadata
• scope to a bucket via .from(name) and to an index via .index(name)
• create / get / list / delete index

Vector data operations (StorageVectorIndexApi):

• put / get / list / query / delete vectors
• parallel scan via listVectors' segmentCount / segmentIndex

Idiomatic Dart choices

• dataType and distanceMetric are enums (VectorDataType, DistanceMetric), parsed leniently (unknown values decode to null for forward compatibility).
• metadataConfiguration is flattened to a nonFilterableMetadataKeys parameter.
• Batch sizes (put/delete 1-500) and parallel-scan segments (1-16) are validated client-side with ArgumentError.

Tests

• 20 self-contained unit tests using the mock HTTP client, verifying request URLs/bodies and response parsing for every operation, plus the validation paths.

Compliance matrix

• All 14 storage.vector_buckets.* features set to implemented with every new public symbol registered. The symbol check and compliance-file validator both pass locally.

Closes SDK-1307.