fix(util): match parseEnv dotenv semantics

[andrewtdiz]

Fixes #3423

Summary:

• Tighten util.parseEnv argument validation to throw ERR_INVALID_ARG_TYPE for non-string content.
• Match Node dotenv parsing semantics for null-prototype results, sorted keys, export prefixes, inline comments, duplicate keys, empty values, invalid names, and multiline quoted values.
• Add a parity fixture covering the Node semantics that were missing from the existing parseEnv smoke test.

Why this PR is scoped to one issue:

• The issue is specifically about node:util.parseEnv availability/behavior. process.loadEnvFile and shared dotenv loading behavior are covered separately.

Verification:

• RUSTC_WRAPPER= ./run_parity_tests.sh --suite node-suite --module util --filter parse-env
• RUSTC_WRAPPER= cargo build --release -p perry -p perry-runtime -p perry-stdlib
• RUSTC_WRAPPER= cargo test -p perry-runtime util_parse_env
• RUSTC_WRAPPER= cargo test -p perry-codegen --test manifest_consistency
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh

Known limitations / non-goals:

• This does not change process.loadEnvFile path handling or environment mutation behavior.

[andrewtdiz]

Fixed the cargo-test failure on this branch by applying the ReflectIsExtensible stable-hash tag correction needed while current mainline still has the duplicate RegExpEscape tag.

Local checks passed: CARGO_TARGET_DIR=/root/perry/target cargo test -p perry-hir stable_hash::tests::expr_variant_stable_hash_tags_are_unique and git diff --check.