api: structured outputs - chat endpoint (#7900)

Adds structured outputs to chat endpoint
---------

Co-authored-by: Michael Yang <mxyng@pm.me>
Co-authored-by: Hieu Nguyen <hieunguyen1053@outlook.com>

llama/llama_test.go

@@ -1 +1,70 @@
 package llama
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestJsonSchema(t *testing.T) {
+	testCases := []struct {
+		name     string
+		schema   JsonSchema
+		expected string
+	}{
+		{
+			name: "empty schema",
+			schema: JsonSchema{
+				Type: "object",
+			},
+			expected: `array ::= "[" space ( value ("," space value)* )? "]" space
+boolean ::= ("true" | "false") space
+char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
+decimal-part ::= [0-9]{1,16}
+integral-part ::= [0] | [1-9] [0-9]{0,15}
+null ::= "null" space
+number ::= ("-"? integral-part) ("." decimal-part)? ([eE] [-+]? integral-part)? space
+object ::= "{" space ( string ":" space value ("," space string ":" space value)* )? "}" space
+root ::= object
+space ::= | " " | "\n" [ \t]{0,20}
+string ::= "\"" char* "\"" space
+value ::= object | array | string | number | boolean | null`,
+		},
+		{
+			name: "invalid schema with circular reference",
+			schema: JsonSchema{
+				Type: "object",
+				Properties: map[string]any{
+					"self": map[string]any{
+						"$ref": "#", // Self reference
+					},
+				},
+			},
+			expected: "", // Should return empty string for invalid schema
+		},
+		{
+			name: "schema with invalid type",
+			schema: JsonSchema{
+				Type: "invalid_type", // Invalid type
+				Properties: map[string]any{
+					"foo": map[string]any{
+						"type": "string",
+					},
+				},
+			},
+			expected: "", // Should return empty string for invalid schema
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := tc.schema.AsGrammar()
+			if !strings.EqualFold(strings.TrimSpace(result), strings.TrimSpace(tc.expected)) {
+				if diff := cmp.Diff(tc.expected, result); diff != "" {
+					t.Fatalf("grammar mismatch (-want +got):\n%s", diff)
+				}
+			}
+		})
+	}
+}