feat(tool/Postgresql): Set default host and port for Postgresql source (#1927)

### Description

This pull request addresses issue #41 by implementing default connection
values for the Postgresql data source. When a user does not specify a
host or port, the connection will now automatically default to
`localhost:5432`.

---

### Detailed Changes

- **docs/en/reference/prebuilt-tools.md**
   -updated the env variables host and port to optional in the doc

- **internal/prebuiltconfigs/tools/postgres.yaml**
- Added the default values to host and port. Specifically, they are now
`host: ${POSTGRES_HOST:localhost}` and `port: ${POSTGRES_PORT:5432}`.
This allows the configuration loader to directly use `"localhost"` or
`"5432"` as a fallback if the `POSTGRES_HOST` or `POSTGRES_PORT`
environment variables are not set.

---

### PR Checklist

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🛠️ **Fixes** #41

---------

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

docs/en/reference/prebuilt-tools.md

@@ -489,8 +489,8 @@ details on how to connect your AI tools (IDEs) to databases via Toolbox and MCP.
 
 *   `--prebuilt` value: `postgres`
 *   **Environment Variables:**
-    *   `POSTGRES_HOST`: The hostname or IP address of the PostgreSQL server.
-    *   `POSTGRES_PORT`: The port number for the PostgreSQL server.
+    *   `POSTGRES_HOST`: (Optional) The hostname or IP address of the PostgreSQL server.
+    *   `POSTGRES_PORT`: (Optional) The port number for the PostgreSQL server.
     *   `POSTGRES_DATABASE`: The name of the database to connect to.
     *   `POSTGRES_USER`: The database username.
     *   `POSTGRES_PASSWORD`: The password for the database user.

internal/prebuiltconfigs/tools/postgres.yaml

@@ -15,8 +15,8 @@
 sources:
     postgresql-source:
         kind: postgres
-        host: ${POSTGRES_HOST}
-        port: ${POSTGRES_PORT}
+        host: ${POSTGRES_HOST:localhost}
+        port: ${POSTGRES_PORT:5432}
         database: ${POSTGRES_DATABASE}
         user: ${POSTGRES_USER}
         password: ${POSTGRES_PASSWORD}