Trailing newline changes parse result for explicit key with `>` value

[kehoecj]

Describe the bug

yaml.Unmarshal produces different results for ? >0 depending on whether a trailing newline is present. Without a newline it parses successfully as a mapping with key >0. With a newline it fails, interpreting >0 as a block scalar header.

To Reproduce

package main

import (
	"fmt"
	goyaml "github.com/goccy/go-yaml"
)

func main() {
	var v1, v2 any

	// Without trailing newline — succeeds
	err1 := goyaml.Unmarshal([]byte("? >0"), &v1)
	fmt.Printf("without newline: %v (err: %v)\n", v1, err1)

	// With trailing newline — fails
	err2 := goyaml.Unmarshal([]byte("? >0\n"), &v2)
	fmt.Printf("with newline:    %v (err: %v)\n", v2, err2)
}

Output:

without newline: map[>0:<nil>] (err: <nil>)
with newline:    map[:<nil>] (err: [1:3] invalid header option: "0"
>  1 | ? >0
         ^
)

Expected behavior

Both inputs should parse identically. >0 here follows the ? explicit key indicator and should be treated as a plain scalar value, not a block scalar header. A trailing newline should not change how the token is classified.

Version Variables

• Go version: 1.26.3
• go-yaml's Version: v1.19.2

Additional context

We encountered this while building a YAML formatting tool that appends a final newline to files (standard practice). Files containing explicit key indicators with values that resemble block scalar headers become unparseable after the newline is added.