feat(tool/mysql): Set default host and port for MySQL source (#1922)

## Description

This pull request addresses issue
[#29](https://github.com/gemini-cli-extensions/mysql/issues/29) by
implementing default connection values for the MySQL data source. When a
user does not specify a host or port, the connection will now
automatically default to localhost:3306.

### Detailed Changes

1. **internal/prebuiltconfigs/tools/mysql.yaml**
- Added the default values to host and port.Specifically, they are now
host: ${MYSQL_HOST:localhost} and port: ${MYSQL_PORT:3306}.This allows
the configuration loader to directly use "localhost" or "3306" as a
fallback if the MYSQL_HOST or MYSQL_PORT environment variables are not
set.

**Testing and Validation**
To ensure the changes work correctly and are compatible with MySQL-like
databases, the feature was manually tested against a MariaDB instance
running in a Docker container.

The testing process involved the following steps:

1. A local testdb database with a products table was created in the
MariaDB container.
2. For Gemini CLI integration, the ~/.gemini/settings.json file was
configured to point to a local build of the toolbox executable.
3. The toolbox was launched in UI mode using the **_./toolbox
--prebuilt=mysql --ui_** command.
4. Two connection scenarios were tested to validate the new logic.

**Test 1: Default Connection (Host/Port Unset)**

The MYSQL_HOST and MYSQL_PORT environment variables were unset. The
application correctly defaulted to localhost:3306 and successfully
connected to the testdb database.
<img width="2087" height="1102" alt="Screenshot 2025-11-11 11 31 45 PM"
src="https://github.com/user-attachments/assets/a8ef5f92-eaca-472f-a0df-e2b3c0c027da"
/>

**Test 2: Explicit Connection (Host/Port Set)**

The MYSQL_HOST and MYSQL_PORT environment variables were explicitly set.
The application correctly used these values, overriding the defaults and
establishing a successful connection.
<img width="2073" height="958" alt="Screenshot 2025-11-11 6 12 44 PM"
src="https://github.com/user-attachments/assets/4b9b8838-091f-4c78-9e3b-97768323693c"
/>

**Result:**

In both scenarios, the list_tables prebuilt tool was executed via the
Toolbox UI, which successfully returned the products table from the
testdb database, confirming the changes work as expected.

Screenshot of list_tables execution from the Toolbox UI:
<img width="2251" height="1240" alt="result"
src="https://github.com/user-attachments/assets/f1c5372d-acc0-4551-af2d-fa1ee4b228d7"
/>

## 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)
- [x] 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
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes [#29](https://github.com/gemini-cli-extensions/mysql/issues/29)

---------

Co-authored-by: prernakakkar-google <158031829+prernakakkar-google@users.noreply.github.com>

internal/prebuiltconfigs/tools/mysql.yaml

@@ -15,8 +15,8 @@
 sources:
   mysql-source:
     kind: mysql
-    host: ${MYSQL_HOST}
-    port: ${MYSQL_PORT}
+    host: ${MYSQL_HOST:localhost}
+    port: ${MYSQL_PORT:3306}
     database: ${MYSQL_DATABASE}
     user: ${MYSQL_USER}
     password: ${MYSQL_PASSWORD}