umqtt.simple: Encode subscribe Remaining Length as VBI.

[pablogventura]

Summary

Fixes #969. subscribe (and unsubscribe) packed Remaining Length with
struct.pack_into("!BH", ...), so values above 127 overflow a single byte
and long topics fail on the wire.

MQTT 3.1.1 section 2.2.3 requires a Variable Byte Integer. This PR encodes
Remaining Length that way for subscribe/unsubscribe, sharing a small
_encode_len helper with connect and publish.

Testing

• Unix port: micropython micropython/umqtt.simple/test_umqtt_simple.py
  (mock socket, no broker):
  • short topic (single-byte Remaining Length)
  • subscribe with topic length 123 (Remaining Length 128 -> 82 80 01)
  • unsubscribe with topic length 124 (Remaining Length 128 -> a2 80 01)
• simple.mpy via mpy-cross vs upstream/master: 2512 -> 2515 bytes (+3).

Trade-offs and Alternatives

• Subscribe and unsubscribe share _send_subunsub; Remaining Length encoding
  is factored into _encode_len and reused by connect/publish to keep .mpy
  size close to master.
• unsubscribe is included because it had the same one-byte Remaining Length
  bug.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.

[dpgeorge]

Thanks for the fix.

Can you try and reduce compiled .mpy code size here? Eg by creating a helper function, or even combining subscribe and unsubscribe into one helper function.

[pablogventura]

Thanks. I combined the shared path into _send_subunsub() in 51bbbaa (subscribe/unsubscribe now just set type, ACK opcode, and optional QoS).

simple.mpy sizes with mpy-cross vs upstream/master:

Version	bytes	vs master	vs previous commit
master	2512	-	-
first fix (duplicated VBI)	2658	+146	-
_send_subunsub helper	2568	+56	-90

Unix mock tests still pass.

[pablogventura]

Thanks again. Further size pass in the latest commit: shared _encode_len() used by connect, publish, and _send_subunsub.

simple.mpy sizes with mpy-cross (compiled as simple.py in umqtt/):

Version	bytes	vs master	vs previous
master	2512	-	-
first fix (duplicated VBI)	2658	+146	-
_send_subunsub	2568	+56	-90
shared _encode_len	2515	+3	-53

Unix mock tests still pass.

[pablogventura]

Fixed the ruff E711 failure: _send_subunsub now uses ack_n > 3 instead of comparing qos to None (subscribe ACK is 4 bytes, unsubscribe is 3).

.mpy size unchanged: 2515 bytes (+3 vs master).