List the MCUboot update image for an nRF52 build that also has a UF2

[bdraco]

What does this implement/fix?

Lists zephyr/app_update.bin in firmware/get_binaries whenever it is on disk.

An nRF52 build with the mcumgr OTA chains MCUboot behind the Adafruit bootloader, so it produces a UF2 and the update image. ESPHome's get_download_types listed the image only when there is no UF2, so the frontend's MCUboot updates had nothing to download on those boards. That is fixed in esphome/esphome#19769, which is in dev and not in a release yet; this covers ESPHome 2026.9 and older.

Same approach as firmware.elf, which is already appended here.

Related issue or feature (if applicable):

• n/a

Types of changes

• Bugfix (non-breaking change which fixes an issue) — bugfix
• New feature (non-breaking change which adds functionality) — new-feature
• Enhancement to an existing feature — enhancement
• Breaking change (fix or feature that would cause existing functionality to not work as expected) — breaking-change
• Refactor (no behaviour change) — refactor
• Documentation only — docs
• Maintenance / chore — maintenance
• CI / workflow change — ci
• Dependencies bump — dependencies

Frontend coordination

• No frontend change needed
• Companion frontend PR: Add MCUboot OTA for nRF52 over BLE and serial device-builder-frontend#1891

Checklist

• The code change is tested and works locally.
• Pre-commit hooks pass (ruff, codespell, yaml/json/python checks).
• Tests have been added or updated under tests/ where applicable.
• components.index.json / definitions/components/*.json have not been hand-edited (regenerate via script/sync_components.py if a sync is needed).
• Architecture-level changes are reflected in docs/ARCHITECTURE.md and/or docs/API.md.

[codspeed]

Merging this PR will not alter performance

✅ 28 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing nrf52-list-mcuboot-update-image (1830f7f) with main (4e1c866)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[Copilot]

Copilot review overview

🟢 Approval recommended

Only nit-level documentation refinements remain, with no blocking issues.

Review effort: Lite
Findings: None

What changed in this PR

Adds MCUboot zephyr/app_update.bin to firmware downloads when present alongside UF2 artifacts.

Changes:

• Discovers and deduplicates the MCUboot update image.
• Adds coverage for presence, absence, and duplicate scenarios.
• Documents the expanded firmware artifact list.

File	Summary	Final review comments
tests/​controllers/​firmware/​test_get_binaries.py	Tests MCUboot artifact handling.	Two nit comments (1 vote each): shorten docstrings to one-line behavior contracts.
esphome_device_builder/​controllers/​firmware/​download.py	Discovers the MCUboot update image.	One nit comment (1 vote): make the comment state the concise invariant.
docs/​API.md	Documents the new downloadable artifact.	None.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.75%. Comparing base (4e1c866) to head (1830f7f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2852   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         295      295           
  Lines       25562    25565    +3     
=======================================
+ Hits        25500    25503    +3     
  Misses         62       62           

Flag	Coverage Δ
py3.12	99.72% <100.00%> (+<0.01%)	⬆️
py3.14	99.60% <100.00%> (+0.35%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...me_device_builder/controllers/firmware/download.py	100.00% <100.00%> (ø)

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