ISSUE-4108: nfc: fix value-block BusFault and add unit test + test file generator

[herbenderbler]

Summary

This PR fixes a crash during MIFARE Classic write flows when value blocks are encountered (reported as NFC BusFault in issue #4108).

Problem:

During write-condition evaluation, value blocks were flagged as is_value_block = true, but the write/read key type fields in the write context were not initialized on that path. Later logic consumed those fields, which could result in invalid enum usage and a BusFault during NFC write operations.

Affected code path:

• lib/nfc/protocols/mf_classic/mf_classic_poller.c

Solution:

In the value-block branch of mf_classic_handler_check_write_conditions():

• Select a valid key type (KeyA or KeyB) based on read access.
• Initialize both key_type_read and key_type_write before any downstream use.
• If neither key has read access, skip the block safely.
• Set need_halt_before_write = false for value-block flow.
• Transition to ReadBlock and exit early from this handler path to avoid falling through into non-value-block key derivation logic.

Also added:

• Targeted unit test source covering value-block initialization and write-state flow:
  • applications/debug/unit_tests/tests/nfc/mf_classic_write_value_block_test.c
• Helper generator for reproducible NFC test input files:
  • generate_test_nfc_files.py

Verification

Performed

• Built firmware after fix (./fbt firmware_all) to verify integration/compilation.
• Compiled the new unit test source directly with GCC to verify syntax and includes:
  • gcc -c -I. -Ilib -Ifuri -Itargets/f7/furi_hal -Itargets/f7/furi -Iapplications/debug/unit_tests/tests applications/debug/unit_tests/tests/nfc/mf_classic_write_value_block_test.c -o /tmp/test_compile.o
• Manual static path validation of value-block branch to confirm key initialization invariants.

Recommended/Prepared

• Unit test execution in unit_tests app set.
• On-device NFC write validation with MIFARE Classic cards (including value-block scenarios) using generated test files.

Checklist (For Reviewer)

• PR has description of feature/bug or link to Confluence/Jira task
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix

[akrylysov]

I tried building a firmware with this change included and it unfortunately didn't fix the crash. It's not impossible the crash I'm seeing is a different bug.

[herbenderbler]

I tried building a firmware with this change included and it unfortunately didn't fix the crash. It's not impossible the crash I'm seeing is a different bug.

Thanks for that feedback. Let me take another pass at it. How can I replicate the crash/error you received? Would you mind dropping more details in this thread?