fix(runtime): honor Function.prototype.toString in String(fn)

[andrewtdiz]

Summary

• Resolve function toString through %Function.prototype% for closure string coercion so monkeypatches affect String(fn) and new String(fn).
• Reuse the same lookup for native fn.toString() dispatch before falling back to registered source text.
• Add a focused globals parity fixture for plain functions, arrows, and boxed String coercion.

Tests

• cargo check -q -p perry-hir -p perry-runtime (passes with existing warning noise)
• PATH=/root/.npm/_npx/bac97da9607b7ef2/node_modules/.bin:$PATH node --experimental-strip-types test-parity/node-suite/globals/function-tostring-override.ts
• PATH=/root/.npm/_npx/bac97da9607b7ef2/node_modules/.bin:$PATH PERRY_NO_AUTO_OPTIMIZE=1 ./run_parity_tests.sh --suite node-suite --module globals --filter function-tostring
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh

Fixes #4535