fix: reject grammars without properties

[drbh]

This PR is adds a new check to grammar validation that ensures that the grammar contains properties

For a grammar to be valid it needs to 1. be a valid JSON Schema, and 2. have properties that are used to constrain the output.

This PR is related to
pr: #2282
issue: #2280

req/res example

The following request would crash TGI due to outlines failing to parse the grammar, now an error is returned because value does not include properties

curl localhost:3000/generate \
    -X POST \
    -H 'Content-Type: application/json' \
    -d '{
        "inputs": "My name is Olivier and I",
        "parameters": {
            "grammar": {"type": "json", "value": {"abc":"def"}}
            }
        }'

now returns

{
    "error": "Input validation error: grammar is not valid: Grammar must have a 'properties' field",
    "error_type": "validation"
}

[ErikKaum]

Nice 👍 this is the 80% catch that I wasn't able to find!

[ErikKaum]

lgtm 👍