fix(moonshot): strip $ref siblings and collapse tuple items in tool schemas (#27104)

Port from anomalyco/opencode#24730: Moonshot's JSON Schema validator rejects
two shapes that the rest of the JSON Schema ecosystem accepts:

1. $ref nodes with sibling keywords. Moonshot expands the reference before
   validation and then rejects the node if keys like `description`, `type`,
   or `default` appear alongside $ref. MCP-sourced tool schemas commonly
   put a `description` on $ref-typed properties so the model sees the
   field hint — which worked on every provider except Moonshot.

2. Tuple-style `items` arrays (positional element schemas). Moonshot's
   engine requires ONE schema applied to every array element. Common in
   tool schemas generated from Go/Protobuf that model fixed-length arrays
   as `[{type:number}, {type:number}]`.

Repairs applied in `agent/moonshot_schema.py`:

- Rule 3: when a node has `$ref`, return `{"$ref": <value>}` only
  (strip every sibling). The referenced definition still carries its own
  description on the target node, which Moonshot accepts.
- Rule 4: when `items` is a list, collapse to the first element schema
  (falling back to `{}` which is then filled by the generic missing-type
  rule). Preserves `minItems` / `maxItems` / other siblings.

Tests: 10 new cases across TestRefSiblingStripping + TestTupleItems,
plus the existing TestMissingTypeFilled::test_ref_node_is_not_given_synthetic_type
still passes (it asserted plain $ref passes through; now it passes through
as exactly `{"$ref": "..."}` which is strictly compatible).

All 35 tests in test_moonshot_schema.py pass.

agent/moonshot_schema.py

@@ -15,6 +15,18 @@ and MoonshotAI/kimi-cli#1595:
 2. When ``anyOf`` is used, ``type`` must be on the ``anyOf`` children, not
    the parent.  Presence of both causes "type should be defined in anyOf
    items instead of the parent schema".
+3. ``enum`` arrays on scalar-typed nodes may not contain ``null`` or empty
+   strings.  Strip those entries (drop the enum entirely if it becomes empty).
+4. ``$ref`` nodes may not carry sibling keywords.  Moonshot expands the
+   reference before validation and then rejects the node if sibling keys
+   like ``description`` remain on the same node as ``$ref``.  Strip every
+   sibling from ``$ref`` nodes so only ``{"$ref": "..."}`` survives.
+   (Ported from anomalyco/opencode#24730.)
+5. ``items`` may not be a tuple-style array (``items: [schemaA, schemaB]``
+   for positional element schemas).  Moonshot's schema engine requires a
+   single object schema applied to every array element.  Collapse tuple
+   ``items`` to the first element schema (or ``{}`` if the tuple is empty).
+   (Ported from anomalyco/opencode#24730.)
 
 The ``#/definitions/...`` → ``#/$defs/...`` rewrite for draft-07 refs is
 handled separately in ``tools/mcp_tool._normalize_mcp_input_schema`` so it
@@ -66,6 +78,16 @@ def _repair_schema(node: Any, is_schema: bool = True) -> Any:
             }
         elif key in _SCHEMA_LIST_KEYS and isinstance(value, list):
             repaired[key] = [_repair_schema(v, is_schema=True) for v in value]
+        elif key == "items" and isinstance(value, list):
+            # Rule 5: tuple-style ``items`` arrays (positional element
+            # schemas) are not accepted by Moonshot.  Collapse to the
+            # first element schema if present, else to ``{}``.  This
+            # matches opencode's behaviour for moonshotai / kimi models.
+            first = value[0] if value else {}
+            if isinstance(first, dict):
+                repaired[key] = _repair_schema(first, is_schema=True)
+            else:
+                repaired[key] = first
         elif key in _SCHEMA_NODE_KEYS:
             # items / not / additionalProperties: single nested schema.
             # additionalProperties can also be a bool — leave those alone.
@@ -130,6 +152,15 @@ def _repair_schema(node: Any, is_schema: bool = True) -> Any:
             else:
                 repaired.pop("enum")
 
+    # Rule 4: $ref nodes must not have sibling keywords.  Moonshot expands
+    # the reference before validation and then rejects the node if siblings
+    # like ``description`` / ``type`` / ``default`` appear alongside $ref.
+    # The referenced definition still carries its own description on the
+    # target node, which Moonshot accepts.
+    # (Ported from anomalyco/opencode#24730.)
+    if "$ref" in repaired:
+        return {"$ref": repaired["$ref"]}
+
     return repaired