requests: HTTP/1.1 response bodies and max_body limit

[pablogventura]

Summary

I use requests on small boards fairly often, and the current package left me hanging on
chunked responses (raw.read() never finished) and still spoke HTTP/1.0 on the wire. This PR
is the first slice of the HTTP work I outlined in #1118: enough client behaviour for real
servers without pulling in TLS or streaming yet.

What I changed:

• Added requests/_http.py to parse status/headers and read bodies (Content-Length and
  chunked). The full body lands in Response.content so callers do not have to fight the
  socket after the headers.
• Requests now use HTTP/1.1 with Connection: close (no keep-alive in this PR).
• resp.headers is a small Headers mapping with case-insensitive keys.
• Relative Location values on redirects are resolved against the original URL (fixes "requests" does not handle redirection with relative path correctly #931).
• New max_body argument, default 32 KiB; pass max_body=None if you really want no cap.

Closes #1118.

Testing

I started with the existing socket-mock tests and extended them for bodies, chunked encoding,
redirects, and max_body. On my machine:

python3 -S python-ecosys/requests/test_requests.py

All 14 mock tests pass on CPython 3.12.

I also ran the same file on the unix port (build-standard) and added test_requests_live.py
(local TCP server in a thread) plus run-unix-tests.sh — 14 + 4 tests, all green.

On hardware I flashed ESP32_GENERIC (ESP32-D0WD, 4 MiB flash), copied this branch’s package
to /lib/requests (the board still ships an older frozen requests, so tests prefer /lib),
and ran run-esp32-tests.sh (mock + live). Same counts, all pass.

Finally I connected the board to my home WiFi and hit real HTTP endpoints (example.com,
httpbin.org GET + redirect) — status 200, bodies as expected. I have not added HTTPS here;
that stays for a follow-up (#838).

CI in this repo should pick up python-ecosys/requests/test_requests.py with the project’s
MicroPython build; the live/ESP32 scripts are mainly for local regression.

Trade-offs and alternatives

• The entire body is buffered in RAM. That is deliberate for simplicity on microcontrollers; I
  documented it and added max_body so a runaway response does not silently exhaust memory.
• No keep-alive, gzip, or TLS certificate verification in this PR — I plan those separately
  (SECURITY: Requests module HTTPS - no server certificate verification. #838, requests: stream parameter is noop #777).
• Outgoing header order still follows dict insertion order, same as before (I did not sort
  headers for diff stability).

Generative AI

I used generative AI tools while working on this PR (including help polishing this
description). I reviewed all of the code and take responsibility for the changes and the
text above.