refactor: optimize internal string building

[KoreaNirsa]

Summary

This PR optimizes a couple of internal string-building paths without changing existing behavior

• Replace per-byte String.format("%02x", b) usage in JdkSha256HexIdGenerator with Java 17 HexFormat
• Replace string concatenation via stream reduce in MiniMaxApi#getTextContent with Collectors.joining()
• Add a regression test to verify that the SHA-256 based document ID remains stable

Performance benefit

String.format("%02x", b) is relatively expensive for byte-by-byte hex encoding because it goes through formatter parsing and formatting machinery on every byte. HexFormat is purpose-built for byte-to-hex conversion and avoids that repeated formatting overhead

Similarly, string concatenation inside reduce creates intermediate String instances as the stream is accumulated Collectors.joining() uses a mutable accumulation strategy internally, which avoids repeated intermediate concatenations and is the idiomatic collector for joining stream elements