fix(scanner): treat colon as plain scalar content in flow mapping keys

In flow mapping context, a ':' not followed by whitespace, a flow
indicator, or a quote should be part of the plain scalar key, not a
mapping value indicator. This fixes parsing of inputs like
{hello:world: [a,b,c]} where the key is the plain scalar 'hello:world'.

Fixes #832

Author: lawrence3699

decode_test.go

@@ -1288,6 +1288,26 @@ c:
 			source: "42: 100",
 			value:  map[int]any{42: 100},
 		},
+		{
+			source: "{hello:world: [a,b,c]}",
+			value:  map[string]any{"hello:world": []any{"a", "b", "c"}},
+		},
+		{
+			source: "{a:b:c: value}",
+			value:  map[string]any{"a:b:c": "value"},
+		},
+		{
+			source: "{host:port: 8080, name: test}",
+			value:  map[string]any{"host:port": uint64(8080), "name": "test"},
+		},
+		{
+			source: "{a:b: {c: d}}",
+			value:  map[string]any{"a:b": map[string]any{"c": "d"}},
+		},
+		{
+			source: "{a:b: [1,2]}",
+			value:  map[string]any{"a:b": []any{uint64(1), uint64(2)}},
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.source, func(t *testing.T) {

scanner/scanner.go

@@ -983,8 +983,17 @@ func (s *Scanner) scanMapDelim(ctx *Context) (bool, error) {
 	if s.startedFlowMapNum <= 0 && nc != ' ' && nc != '\t' && !s.isNewLineChar(nc) && !ctx.isNextEOS() {
 		return false, nil
 	}
-	if s.startedFlowMapNum > 0 && nc == '/' {
-		// like http://
+	if s.startedFlowMapNum > 0 && len(ctx.buf) > 0 &&
+		nc != ' ' && nc != '\t' && !s.isNewLineChar(nc) &&
+		nc != ',' && nc != '}' && nc != ']' && nc != '[' && nc != '{' &&
+		nc != '"' && nc != '\'' && !ctx.isNextEOS() {
+		// In flow mapping context, when scanning a plain scalar key (buf is non-empty),
+		// ':' is only a mapping value indicator when followed by whitespace,
+		// a flow indicator (',', '}', ']'), a flow collection start ('[', '{'),
+		// a quote ('"', '\''), or end of input.
+		// This allows unquoted keys like 'hello:world' and URLs like 'http://example.com'.
+		// When buf is empty the key was already tokenized (e.g. as a quoted string),
+		// so ':' is unambiguously a mapping value indicator.
 		return false, nil
 	}
 	if s.startedFlowMapNum > 0 {