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.