Add createNamespace(), and prefix + totalCount to listNamespaces()

[rohanshah18]

Problem

Add support for createNamespace() endpoint, and add prefix + totalCount to listNamespaces().

Solution

This PR adds the createNamespace method and enhances listNamespaces with prefix filtering and total count support for both Index and AsyncIndex clients.

Create Namespace:

1. Basic Namespace Creation: Create a namespace explicitly by name:

// Create a namespace explicitly
NamespaceDescription createdNamespace = index.createNamespace("my-namespace");
assertNotNull(createdNamespace);
assertEquals("my-namespace", createdNamespace.getName());

// Verify the namespace was created
ListNamespacesResponse response = index.listNamespaces();
assertTrue(response.getNamespacesList().stream()
    .anyMatch(ns -> ns.getName().equals("my-namespace")));

2. Metadata Schema Support: Create a namespace with a metadata schema to define filterable fields:

// Create a namespace with metadata schema
MetadataSchema schema = MetadataSchema.newBuilder()
    .putFields("genre", MetadataFieldProperties.newBuilder().setFilterable(true).build())
    .putFields("year", MetadataFieldProperties.newBuilder().setFilterable(true).build())
    .build();

NamespaceDescription namespaceWithSchema = index.createNamespace("movies-namespace", schema);
assertNotNull(namespaceWithSchema);
assertEquals("movies-namespace", namespaceWithSchema.getName());

Prefix Filtering: Filter namespaces by prefix to retrieve only namespaces starting with a specific prefix:

// List namespaces with prefix filtering
ListNamespacesResponse response = index.listNamespaces("test-", null, 100);
int totalCount = response.getTotalCount(); // Total namespaces matching "test-" prefix
List<NamespaceDescription> namespaces = response.getNamespacesList();

// Verify all returned namespaces match the prefix
for (NamespaceDescription ns : namespaces) {
    assert ns.getName().startsWith("test-");
}

Total Count: The response includes a totalCount field indicating the total number of namespaces matching the prefix (useful for pagination):

// Get total count even when results are paginated
ListNamespacesResponse response = index.listNamespaces("prod-", null, 10);
int totalCount = response.getTotalCount(); // Total matching namespaces
int returnedCount = response.getNamespacesCount(); // Namespaces in this page (≤ 10)

// Use totalCount to determine if more pages are available
if (totalCount > returnedCount) {
    // More namespaces available via pagination
}

Async Support: Same functionality available for AsyncIndex:

// Create namespace asynchronously
ListenableFuture<NamespaceDescription> future = asyncIndex.createNamespace("async-namespace");
NamespaceDescription createdNamespace = future.get();

// Create namespace with schema asynchronously
ListenableFuture<NamespaceDescription> futureWithSchema = 
    asyncIndex.createNamespace("async-movies", schema);
NamespaceDescription namespace = futureWithSchema.get();

// List namespaces with prefix filtering asynchronously
ListenableFuture<ListNamespacesResponse> listFuture = asyncIndex.listNamespaces("dev-", null, 50);
ListNamespacesResponse response = listFuture.get();
int totalCount = response.getTotalCount();

Note:

• Null or empty prefix values are ignored (no filtering applied)
• The method signature is: listNamespaces(String prefix, String paginationToken, int limit)
• Both synchronous (Index) and asynchronous (AsyncIndex) implementations are included
• Fully backward compatible - existing listNamespaces() methods remain unchanged

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Updated NamespacesTests.java to include:

• createNamespace() call
• prefix and totalCount