Update ADK doc according to issue #1048 - 6

[adk-bot]

  6. **[Feature] Document Spanner `QueryResultMode.DICT_LIST`.**

The Spanner tool now supports returning query results as a list of dictionaries.

Proposed Change:
In docs/tools/built-in-tools.md, update the Spanner section to include information about QueryResultMode.DICT_LIST.

Current state:
The documentation for the Spanner toolset is minimal.

Proposed Change:
Add the following to the Spanner section:

The `execute_sql` tool can now return results as a list of dictionaries. To enable this, set `query_result_mode=QueryResultMode.DICT_LIST` in `SpannerToolSettings`.

```python
from google.adk.tools.spanner.settings import QueryResultMode, SpannerToolSettings
from google.adk.tools.spanner.spanner_toolset import SpannerToolset

tool_settings = SpannerToolSettings(query_result_mode=QueryResultMode.DICT_LIST)
spanner_toolset = SpannerToolset(spanner_tool_settings=tool_settings)

**Reasoning**:
This new feature provides a more convenient way to work with Spanner query results, and it should be documented.

**Reference**:
- `src/google/adk/tools/spanner/settings.py`
- `src/google/adk/tools/spanner/query_tool.py`