Handle modified Enter as newline in multiline linenoise

[ljubomirj]

Summary

This teaches the linenoise multiline editor to insert a literal newline when the terminal reports modified Enter.

Handled cases:

• Meta/Option+Enter as ESC followed by CR or LF, used by macOS terminals and other terminal configurations.
• Kitty keyboard protocol Enter reports such as ESC [ 13 ; modifier u.

The behavior is gated on mlmode, so normal single-line linenoise use continues to ignore those modified Enter sequences.

Validation

• git diff --check upstream/main...HEAD
• make -j8 ds4-agent ds4_agent_test
• ./ds4_agent_test