…on declarations with nested object schemas

Gemini's function-calling API silently rejects function declarations whose
parameter schema carries a `required` field below the top-level object —
e.g. on the `items.properties` of an array, or on a nested property's
own `properties`. The model emits no error event and no content events,
just RUN_STARTED → STATE_SNAPSHOT → RUN_FINISHED, which surfaces as a
frozen agent: the user sends a message, no chart/UI ever renders, no text
ever streams back.

The most common shape that trips this is Zod's `z.array(z.object({...}))`
when the inner object marks fields as required:

```json
{
  "type": "object",
  "required": ["data"],
  "properties": {
    "data": {
      "type": "array",
      "items": {
        "type": "object",
        "properties": {"label": {...}, "value": {...}},
        "required": ["label", "value"]   ← Gemini chokes on this
      }
    }
  }
}
```

This shape is produced by every CopilotKit gen-UI demo that renders
structured data (charts, tables, lists of records). It's also what any
backend tool that takes an array of typed records will generate from
Pydantic / Zod / similar libraries. The bug has been latent since
`_clean_schema_for_genai` was introduced in #1349 — OpenAI-backed
consumers don't hit it because OpenAI's function-calling API accepts
`required` at every nesting level, and simpler ADK demos with flat
schemas don't have nested objects to begin with.

Fix: add a `_top_level` parameter to `_clean_schema_for_genai` (defaults
to True for the outermost call, False on every recursive call). When
False, drop any `required` key. Top-level `required` is preserved —
Gemini does accept it there.

Verified against the CopilotKit `gen-ui-tool-based` demo: before the
fix, `render_bar_chart` calls returned empty SSE streams; after the
fix, TOOL_CALL_START + TOOL_CALL_ARGS containing the structured chart
payload are emitted and the chart renders end-to-end.

Tests: four new regression tests in `TestCleanSchemaForGenai` cover
(1) top-level required preservation, (2) the array-of-objects shape,
(3) nested-object-property required, (4) arbitrary-depth required.
All 41 existing tests in `test_client_proxy_tool.py` continue to pass.