llama: preserve field order in user-defined JSON schemas (#8002)

Previously we decoded and re-encoded JSON schemas during validation,
which served no purpose since json.RawMessage already validates JSON
syntax. Worse, the re-encoding lost field ordering from the original
schema, which affects inference quality during step-by-step reasoning.

While fixing this ordering issue by using json.RawMessage directly,
testing revealed that schema_to_grammar (from llama.cpp) also fails to
preserve field order during grammar generation. This appears to be the
root cause of inference degradation.

This change prevents us from mangling the user's original schema order,
but we still need to address the ordering issue in schema_to_grammar.
That will be a separate change.

Updates #7978

llama/llama.go

@@ -86,12 +86,9 @@ COMPILER inline get_compiler() {
 import "C"
 
 import (
-	"bytes"
 	_ "embed"
-	"encoding/json"
 	"errors"
 	"fmt"
-	"log/slog"
 	"runtime"
 	"runtime/cgo"
 	"slices"
@@ -721,21 +718,10 @@ func (s *SamplingContext) Accept(id int, applyGrammar bool) {
 	C.common_sampler_caccept(s.c, C.llama_token(id), C.bool(applyGrammar))
 }
 
-type JsonSchema struct {
-	Defs       map[string]any `json:"$defs,omitempty"`
-	Properties map[string]any `json:"properties,omitempty"`
-	Required   []string       `json:"required,omitempty"`
-	Title      string         `json:"title,omitempty"`
-	Type       string         `json:"type,omitempty"`
-}
-
-func (js JsonSchema) AsGrammar() string {
-	var b bytes.Buffer
-	if err := json.NewEncoder(&b).Encode(js); err != nil {
-		return ""
-	}
-
-	cStr := C.CString(b.String())
+// SchemaToGrammar converts the provided JSON schema to a grammar. It returns
+// nil if the provided schema is invalid JSON or an invalid JSON schema.
+func SchemaToGrammar(schema []byte) []byte {
+	cStr := C.CString(string(schema))
 	defer C.free(unsafe.Pointer(cStr))
 
 	// Allocate buffer for grammar output with reasonable size
@@ -743,10 +729,10 @@ func (js JsonSchema) AsGrammar() string {
 	buf := make([]byte, maxLen)
 
 	// Call C function to convert schema to grammar
-	length := C.schema_to_grammar(cStr, (*C.char)(unsafe.Pointer(&buf[0])), C.size_t(maxLen))
-	if length == 0 {
-		slog.Warn("unable to convert schema to grammar")
+	n := C.schema_to_grammar(cStr, (*C.char)(unsafe.Pointer(&buf[0])), C.size_t(maxLen))
+	if n == 0 {
+		// preserve nil
+		return nil
 	}
-
-	return string(buf[:length])
+	return buf[:n]
 }