fix(idl): avoid quoted string constant values

[abhinavgautam01]

Fixes #3607

Summary

• Emit string #[constant] values using their actual string contents instead of Rust debug formatting
• Prevent generated IDL / TypeScript types from containing escaped quote wrappers like "\"favor\""
• Add regression coverage for a &str constant in the IDL fixture

Testing

• cargo test -p anchor-syn
• cargo test -p anchor-cli idl_ts_preserves_literal_values
• CARGO_NET_OFFLINE=true target/debug/anchor idl build -o /private/tmp/idl-3607.json
• diff -u tests/idl/idls/idl.json /private/tmp/idl-3607.json
• target/debug/anchor idl type /private/tmp/idl-3607.json > /private/tmp/idl-3607.ts
• rg -n 'seedPrefix|favor|\\"favor\\"' /private/tmp/idl-3607.ts
• git diff --check

[0x4ka5h]

lgtm