fix: non-utf-8 decode error

[nancyc12]

This PR fixes UTF-8 decoding error when processing serial logs containing non-UTF-8 bytes (like 0xff).

Description

PR #666 (Logging Redesign) introduced the write_from_file() method without the errors="ignore" parameter that the previous code used when reading serial logs.
Added encoding="utf-8", errors="ignore" to restore the exact previous behavior.

Resolved issues

[25-12-30 07:58:08]    INFO: (job.py:129)| Running setup_command: tf-setup
[25-12-30 07:58:26]    INFO: (job.py:129)| Running provision_command: tf-provision
[25-12-30 08:11:09]   ERROR: (agent.py:415)| 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte
Traceback (most recent call last):
  File "/srv/testflinger-venv/lib/python3.10/site-packages/testflinger_agent/agent.py", line 377, in process_jobs
    exit_code, exit_event, exit_reason = job.run_test_phase(
  File "/srv/testflinger-venv/lib/python3.10/site-packages/testflinger_agent/job.py", line 194, in run_test_phase
    self.serial_output_handler.write_from_file(serial_log)
  File "/srv/testflinger-venv/lib/python3.10/site-packages/testflinger_agent/handlers.py", line 198, in write_from_file
    data = log.read(chunk_size)
  File "/usr/lib/python3.10/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte
[25-12-30 08:11:10]    INFO: (job.py:129)| Running cleanup_command: tf-cleanup

Documentation

Web service API changes

Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.38%. Comparing base (accf330) to head (93c6454).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #856   +/-   ##
=======================================
  Coverage   71.38%   71.38%           
=======================================
  Files         108      108           
  Lines        9674     9674           
  Branches      860      860           
=======================================
  Hits         6906     6906           
  Misses       2595     2595           
  Partials      173      173           

Flag	Coverage Δ		*Carryforward flag
agent	72.85% <100.00%> (ø)
cli	87.90% <ø> (ø)		Carriedforward from accf330
device	55.21% <ø> (ø)		Carriedforward from accf330
server	87.71% <ø> (ø)		Carriedforward from accf330

*This pull request uses carry forward flags. Click here to find out more.

Components	Coverage Δ
Agent	72.85% <100.00%> (ø)
CLI	87.90% <ø> (ø)
Common	∅ <ø> (∅)
Device Connectors	55.21% <ø> (ø)
Server	87.71% <ø> (ø)

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[kevinyehk]

LGTM +1