feat(preview): add private_host, source_collection, source_backup_id, cmek_id to PreviewIndexModel

## Purpose

Four optional fields present in backend GlobalIndexMetadata were not
exposed on PreviewIndexModel, so callers had no way to read them even
though msgspec silently discarded the values.

## Solution

Added all four as optional fields with `= None` defaults after `tags`:
- `private_host` — private-network data-plane host when private endpoints are configured
- `source_collection` — collection this index was created from (if any)
- `source_backup_id` — backup this index was created from (if any)
- `cmek_id` — customer-managed encryption key ID (if any)

Updated `__repr__` and `_repr_pretty_` to conditionally include
`private_host` when set. Added five unit tests covering all four fields
present, all absent, partial, and repr rendering.

Author: jhamon

pinecone/preview/models/indexes.py

@@ -38,6 +38,14 @@ class PreviewIndexModel(Struct, kw_only=True):
         read_capacity: Read capacity configuration, or ``None`` for the
             default on-demand mode.
         tags: User-defined key-value tags, or ``None``.
+        private_host: Private-network hostname for the index data plane, or
+            ``None`` when private endpoints are not configured for the project.
+        source_collection: Name of the collection this index was created from,
+            or ``None`` if not created from a collection.
+        source_backup_id: ID of the backup this index was created from, or
+            ``None`` if not created from a backup.
+        cmek_id: ID of the customer-managed encryption key protecting this
+            index, or ``None`` if CMEK is not configured.
     """
 
     name: str
@@ -48,6 +56,10 @@ class PreviewIndexModel(Struct, kw_only=True):
     host: str | None = None
     read_capacity: PreviewReadCapacity | None = None
     tags: dict[str, str] | None = None
+    private_host: str | None = None
+    source_collection: str | None = None
+    source_backup_id: str | None = None
+    cmek_id: str | None = None
 
     def __repr__(self) -> str:
         dep_name = type(self.deployment).__name__.replace("Deployment", "")
@@ -62,6 +74,8 @@ def __repr__(self) -> str:
             parts.append(f"schema_fields={len(self.schema.fields)}")
         if self.tags:
             parts.append(f"tags={len(self.tags)} items")
+        if self.private_host is not None:
+            parts.append(f"private_host={self.private_host!r}")
         return f"PreviewIndexModel({', '.join(parts)})"
 
     def __dir__(self) -> list[str]:
@@ -94,6 +108,9 @@ def _repr_pretty_(self, p: Any, cycle: bool) -> None:
             if self.tags:
                 p.breakable()
                 p.text(f"tags={self.tags!r},")
+            if self.private_host is not None:
+                p.breakable()
+                p.text(f"private_host={self.private_host!r},")
 
     def _repr_html_(self) -> str:
         """Jupyter notebook HTML representation."""

tests/unit/preview/test_preview_index_model.py

@@ -0,0 +1,74 @@
+"""Unit tests for PreviewIndexModel optional response fields."""
+
+from __future__ import annotations
+
+import msgspec
+
+from pinecone.preview.models.indexes import PreviewIndexModel
+
+_BASE_RAW = (
+    b'{"name":"idx","host":"idx.svc.pinecone.io",'
+    b'"status":{"state":"Ready","ready":true},'
+    b'"schema":{"fields":{}},'
+    b'"deployment":{"deployment_type":"managed","cloud":"aws","region":"us-east-1"},'
+    b'"deletion_protection":"disabled"'
+)
+
+
+def test_preview_index_model_optional_response_fields() -> None:
+    raw = (
+        b'{"name":"idx","host":"idx.svc.pinecone.io",'
+        b'"status":{"state":"Ready","ready":true},'
+        b'"schema":{"fields":{}},'
+        b'"deployment":{"deployment_type":"managed","cloud":"aws","region":"us-east-1"},'
+        b'"deletion_protection":"disabled",'
+        b'"private_host":"idx.svc.private.pinecone.io",'
+        b'"source_collection":"my-collection",'
+        b'"source_backup_id":"bkp-abc123",'
+        b'"cmek_id":"cmek-xyz789"}'
+    )
+    model = msgspec.json.decode(raw, type=PreviewIndexModel)
+    assert model.private_host == "idx.svc.private.pinecone.io"
+    assert model.source_collection == "my-collection"
+    assert model.source_backup_id == "bkp-abc123"
+    assert model.cmek_id == "cmek-xyz789"
+
+
+def test_preview_index_model_optional_response_fields_absent() -> None:
+    """Fields are None when absent from the response."""
+    raw = _BASE_RAW + b"}"
+    model = msgspec.json.decode(raw, type=PreviewIndexModel)
+    assert model.private_host is None
+    assert model.source_collection is None
+    assert model.source_backup_id is None
+    assert model.cmek_id is None
+
+
+def test_preview_index_model_repr_includes_private_host_when_set() -> None:
+    raw = _BASE_RAW + b',"private_host":"idx.svc.private.pinecone.io"}'
+    model = msgspec.json.decode(raw, type=PreviewIndexModel)
+    assert "private_host=" in repr(model)
+    assert "idx.svc.private.pinecone.io" in repr(model)
+
+
+def test_preview_index_model_repr_omits_private_host_when_absent() -> None:
+    raw = _BASE_RAW + b"}"
+    model = msgspec.json.decode(raw, type=PreviewIndexModel)
+    assert "private_host" not in repr(model)
+
+
+def test_preview_index_model_partial_optional_fields() -> None:
+    """Only some optional fields present — the rest remain None."""
+    raw = (
+        b'{"name":"idx","host":"idx.svc.pinecone.io",'
+        b'"status":{"state":"Ready","ready":true},'
+        b'"schema":{"fields":{}},'
+        b'"deployment":{"deployment_type":"managed","cloud":"aws","region":"us-east-1"},'
+        b'"deletion_protection":"disabled",'
+        b'"source_collection":"col-1"}'
+    )
+    model = msgspec.json.decode(raw, type=PreviewIndexModel)
+    assert model.source_collection == "col-1"
+    assert model.private_host is None
+    assert model.source_backup_id is None
+    assert model.cmek_id is None