Update ADK doc according to issue #1048 - 6 - Spanner QueryResultMode.DICT_LIST.**

[adk-bot]

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)

[jcwriter74]

Already added in https://github.com/google/adk-docs/pull/1055/changes#:~:text=%2B-,The%20%60execute_sql%60%20tool%20can%20now%20return%20results%20as%20a,%60%60%60,-250

[jcwriter74]

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

[jcwriter74]

• Changes here merged with PR 1898

[zyantw]

Closed with: #2183