Add BinaryWriter benchmarks for _useFastUtf8 true and false code paths

[Copilot]

• Analyze the _useFastUtf8 condition in BinaryWriter (true for UTF-8 encoding, false for non-UTF-8)
• Review existing benchmark coverage (only covers _useFastUtf8 = true path via default UTF-8 encoding, ASCII only)
• Add benchmarks to cover both _useFastUtf8 = true and _useFastUtf8 = false paths
  • BinaryWriterExtendedTests: Added WriteNonAsciiCharArray and WriteNonAsciiString benchmarks (UTF-8, _useFastUtf8 = true with multi-byte chars)
  • BinaryWriterUnicodeEncodingTests: New class using Encoding.Unicode for Write(string) and Write(char[]) (_useFastUtf8 = false)
  • BinaryWriterUnicodeEncodingCharTests: New class using Encoding.Unicode for Write(char) (_useFastUtf8 = false)
• Add comments explaining the significance of '\u00E0' character selection (2-byte UTF-8 encoding)
• Run code review (no issues found)
• Run CodeQL security check (timed out - expected for benchmark-only changes with no security implications)

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.