Add behavioral TS tests for stdlib IO and framework APIs

[TheHypnoo]

Follow-up to #694.

Goal

Convert the stdlib IO, network, stream, and framework FFI inventory into behavioral TypeScript tests.

Scope

Start from test-files/test_ffi_surface_stdlib_io.ts and move coverage entries into focused tests for real IO behavior.

Suggested areas:

• fetch, Request, Response, Headers, Blob
• HTTP and HTTPS client helpers
• framework request/response helpers
• multipart parsing
• streams and stream subclass behavior
• net, TLS, WebSocket, and worker threads
• readline where deterministic enough for CI

Acceptance criteria

• Tests use local deterministic fixtures where possible, such as loopback HTTP/TCP servers.
• Network-dependent behavior avoids external services unless explicitly marked or skipped.
• Converted entries are removed from test_ffi_surface_stdlib_io.ts by running ./test-coverage/regen_ts_surface_inventory.py.
• ./run_parity_tests.sh --filter <name> or the appropriate dedicated script passes for each added fixture.
• ./test-coverage/audit.sh --markdown still reports full TypeScript FFI surface coverage.