tools: ignore braces inside JSON strings when detecting tool call end (#16937)

Parser.done() counted the tag's open/close characters ({}, []) without
tracking JSON string context, so a streamed tool call whose string
argument value contained a closing brace or bracket (e.g.
{"code": "if (x) { y }"}) was treated as complete too early and flushed to
the user as plain text instead of being parsed as a tool call.

findArguments() in the same file already tracks string context; apply the
same handling in done() so open/close characters inside string values are
ignored.

tools/tools.go

@@ -358,7 +358,23 @@ func (p *Parser) done() bool {
 	}
 
 	var count int
+	var inString, escaped bool
 	for _, c := range p.buffer {
+		if escaped {
+			escaped = false
+			continue
+		}
+		if c == '\\' {
+			escaped = true
+			continue
+		}
+		if c == '"' {
+			inString = !inString
+			continue
+		}
+		if inString {
+			continue
+		}
 		if c == byte(open) {
 			count++
 		} else if c == byte(close) {

tools/tools_test.go

@@ -842,6 +842,24 @@ func TestDone(t *testing.T) {
 			buffer: []byte("[]"),
 			want:   true,
 		},
+		{
+			name:   "json close brace inside string",
+			tag:    "{",
+			buffer: []byte("{\"note\": \"a}b\""),
+			want:   false,
+		},
+		{
+			name:   "json complete with brace inside string",
+			tag:    "{",
+			buffer: []byte("{\"note\": \"a}b\"}"),
+			want:   true,
+		},
+		{
+			name:   "list close bracket inside string",
+			tag:    "[",
+			buffer: []byte("[{\"note\": \"x]y\"}"),
+			want:   false,
+		},
 	}
 
 	for _, tt := range tests {