[codex] fix string literal escapes

[justinjoy]

Summary

Fixes #924 by teaching the parser lexer to keep scanning through supported string escapes and by decoding those escapes when materializing string token values.

Root Cause

scan_string() stopped at the first " byte without considering a preceding backslash, so an input such as "a\"b" was split early and the full program parse failed. The parser also had its own raw quote-stripping path, so token decoding had no central unescape behavior.

Changes

• Recognize \" and \\ while scanning string literals.
• Decode \" to " and \\ to \ in wl_parser_lexer_token_to_string().
• Reuse lexer string decoding from parser AST string value construction.
• Add lexer and parser regression coverage for escaped quotes and backslashes.

Validation

• meson setup build-codex --wipe -Dtests=true
• meson test -C build-codex wirelog:lexer wirelog:parser wirelog:string_parser
• git diff --cached --check