wip: use mcumgr params for serial transports

[JPHutchins]

Related to #73

This is a breaking change.

I want to setup a simple Zephyr repo to produce test FWs. I'd like to include some QEMU builds so that we can setup automated regression testing for this sorta thing.

Thank you for the research, @joerchan!

[Copilot]

Pull Request Overview

This PR refactors the SMPSerialTransport class to use mcumgr parameters for configuring serial transport buffers. It introduces a new fragmentation strategy pattern with Auto() and BufferParams classes, replacing the previous direct parameter approach.

• Replaces direct constructor parameters with a fragmentation_strategy parameter that accepts either Auto() or BufferParams
• Adds automatic buffer configuration based on server's MCUMGR_PARAM buffer size
• Updates all test cases and examples to use the new API

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
smpclient/transport/serial.py	Refactors constructor to use fragmentation strategy pattern and adds server buffer size initialization
tests/test_smp_serial_transport.py	Updates tests for new constructor API and adds tests for Auto/BufferParams modes
tests/test_smp_client.py	Updates test transport creation to use new BufferParams syntax
examples/usb/upgrade.py	Updates example code to use new BufferParams constructor pattern

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[JPHutchins]

Great progress! We now have a native_sim fixture that we can use to test stuff out: https://github.com/intercreate/smp-server-fixtures/actions/runs/18636709577

We can always add QEMU in the future - in fact, that would be required to test image management firmware upgrades with MCUBoot - but the native_sim target made this really easy to start out.

In the long run, it probably makes sense to matrix a bunch of configs with twister and then commit the zip to this repo for integration testing.

native_sim terminal:

./build/smp-server/zephyr/zephyr.exe
uart connected to pseudotty: /dev/pts/15
*** Booting Zephyr OS build v4.2.0-6166-g8a72d776c5af ***
[00:00:00.000,000] <inf> littlefs: LittleFS version 2.11, disk version 2.1
[00:00:00.000,000] <inf> littlefs: FS at flash-controller@0:0xfc000 is 4 0x1000-byte blocks with 512 cycle
[00:00:00.000,000] <inf> littlefs: partition sizes: rd 16 ; pr 16 ; ca 64 ; la 32
[00:00:00.000,000] <inf> smp_sample: build time: Oct 19 2025 14:56:16

smpmgr terminal:

smpmgr --port /dev/pts/15 statistics list
⠋ Connecting to /dev/pts/15... OK
⠋ Waiting for response to ListOfGroups... OK
             Statistics Groups
┏━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┓
┃ Group Name           ┃ Number of Groups ┃
┡━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━┩
│ flash_sim_stats      │ 3                │
│ flash_sim_thresholds │ 3                │
│ smp_svr_stats        │ 3                │
└──────────────────────┴──────────────────┘

[JPHutchins]

I think that more optimized defaults should be added. For example, I think that MCUBoot default is 128 * 8, Zephyr main is 128 * 2, etc. Allows the user to opt out of MCUmgr params negotiation and still get maximum throughput.

[JPHutchins]

I think that we dropped 3.9 support, so this should be replaced with exhaustive match case.