feat(scenarios): add firmware-update-failure scenario

[YANGCHUNHONG3000]

Summary

Add irmware-update-failure scenario - the failing counterpart to #133.
Station boots, reports firmware Downloading, then InstallFailed within minutes.

Changes

File	Change
packages/toolkit/src/scenarios/scenarios/firmware-update-failure.ts	New scenario file
packages/toolkit/src/scenarios/index.ts	Register scenario + add to registry
packages/toolkit/src/scenarios/index.test.ts	Add tests + update count to 17
README.md	Update scenario count to 17
packages/toolkit/README.md	Update scenario count to 17
ests/external-fixture/test.mjs	Update scenario count to 17
.changeset/*.md	Minor bump for @ocpp-debugkit/toolkit

Verification

• Station ID: CS-SYNTHETIC-019
• Expected failure: FIRMWARE_UPDATE_FAILURE
• Trace fits within 5 minutes (no heartbeat rule interference)

[YANGCHUNHONG3000]

Apologies for submitting with a minor changeset instead of patch — just caught your message from yesterday. I've amended the commit to fix it. Thanks for the guidance, and for assigning #138 in the first place. PR should be good now.

[sepehr-safari]

Reviewed and merging. The scenario itself is right: I ran its trace through the
detection engine and it fires FIRMWARE_UPDATE_FAILURE and nothing else, the
station ID and the five-minute window are correct, and the changeset is a patch.
Nice work, and thanks for catching the changeset size yourself.

I made two small fixes and pushed them to your branch rather than send you round
again, so your commit is untouched underneath.

• The README feature list bumped the detection-rule count to 17 along with the
  scenario count. Those are two different numbers: a scenario is not a rule, and
  there are still 16 rules. I put that line back to 16 and left the scenario count
  at 17.
• pnpm format on the new file, same one-line collapse as last time.

Two things for next time, both mechanical. Run pnpm format from the repo before
pushing, since the repo's Prettier config is what CI checks first. And branch new
work off main, not off a previous feature branch. This branch was cut from the
firmware-update-success branch before it merged, so it carried commits that were
already on main, which is what made it show as conflicting. I sorted that out by
syncing with main; nothing you need to do.

Going in now. That is two scenarios from you, and it takes the corpus to 17.