Add behavioral TS tests for stdlib external integrations

[TheHypnoo]

Follow-up to #694.

Goal

Convert the stdlib external integration FFI inventory into behavioral TypeScript tests with deterministic local fixtures where possible.

Scope

Start from test-files/test_ffi_surface_stdlib_integrations.ts and move coverage entries into focused tests.

Suggested areas:

• crypto and WebCrypto-compatible helpers
• JWT signing, verification, and decoding with fixed keys/payloads
• bcrypt and argon2 deterministic verify/hash-shape checks
• Cheerio and Sharp with local fixtures
• ethers helpers with fixed inputs
• MongoDB, MySQL, PostgreSQL, Redis, and Nodemailer using local services, mocks, or clearly separated opt-in tests

Acceptance criteria

• Pure or deterministic helpers get normal parity tests.
• Service-backed integrations use local containers/services, mocks, or documented opt-in scripts rather than external network dependencies.
• Non-deterministic outputs are normalized or tested by shape/invariant.
• Converted entries are removed from test_ffi_surface_stdlib_integrations.ts by running ./test-coverage/regen_ts_surface_inventory.py.
• ./test-coverage/audit.sh --markdown still reports full TypeScript FFI surface coverage.