feat(tools/mssql-list-tables): Add new tool for mssql (#1433)

## Description

---
This pull request introduces a new custom tool kind `mssql-list-tables`
that allows users to list tables within a SQL server database.

### Example Configuration

```yaml
tools:
  list_tables:
    kind: mssql-list-tables
    source: mssql-source
    description: Use this tool to retrieve schema information for all or specified tables. Output format can be simple (only table names) or detailed.
```

### Example Request
``` 
curl -X POST http://127.0.0.1:5000/api/tool/list_tables/invoke \
-H "Content-Type: application/json" \
-d '{
    "table_names": "users",
    "output_format": "simple"
}'
```
## PR Checklist

---
> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

---------

Co-authored-by: Averi Kitsch <akitsch@google.com>

docs/en/resources/sources/mssql.md

@@ -23,6 +23,9 @@ amount of data through a structured format.
 - [`mssql-execute-sql`](../tools/mssql/mssql-execute-sql.md)  
   Run parameterized SQL Server queries in SQL Server.
 
+- [`mssql-list-tables`](../tools/mssql/mssql-list-tables.md)  
+  List tables in a SQL Server database.
+
 ## Requirements
 
 ### Database User