[19.0][FIX] base_rest_pydantic: remove duplicate installable key

[fsmw]

Issue: #612

Problem:
The __manifest__.py contained a duplicate installable key. One instance was incorrectly nested inside the external_dependencies dict, making the manifest invalid according to the standard Odoo schema.

Fix:

• Removed the nested installable key from external_dependencies
• Moved the remaining installable key to the root dict with proper double quotes
• Bumped version from 18.0.1.0.2 to 18.0.1.0.3

Before:

"external_dependencies": {
    "python": [
        "pydantic>=2.0.0",
    ],
    "installable": False,
},
    "installable": False,
}

After:

"external_dependencies": {
    "python": [
        "pydantic>=2.0.0",
    ],
},
"installable": False,
}

Fixes #612