refactor: overhaul erpnext_sync and add test suite

[MhmdSinanKT]

Summary

Refactors the core sync script for maintainability and correctness, adds
get_shift_type_device_mapping() to fetch shift-device configuration
directly from ERPNext at startup, and introduces a full test suite with
both unit and live integration tests.

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Changes

erpnext_sync.py — Refactor

New functionality

• get_shift_type_device_mapping(): fetches Shift Type → device mappings
  from ERPNext at startup instead of requiring the user to manually maintain
  shift_type_device_mapping in local_config.py. Config is still
  supported for backward compatibility.

Bug fixes

• send_shift_sync_to_erpnext was double-encoding the request body
  (data=json.dumps(data) → json=payload), causing silent failures on
  the ERPNext side.
• _safe_parse_dt failed to parse timestamps stored via str(datetime),
  which produces no microseconds. This caused update_shift_last_sync_timestamp
  to silently skip syncing for any device whose pull timestamp landed on a
  whole second.
• _find_resume_index had an off-by-one risk in the original
  index_of_last + 1 pattern; replaced with a cleaner direct index return.

Refactors

• Extracted _auth_headers() — single place to manage API credentials format
• Extracted _resolve_punch_direction() — punch AUTO logic out of the loop
• Extracted _find_resume_index() — resume logic now independently testable
• Extracted _build_allowlisted_errors() — replaces module-level procedural block
• os.path.join replaces manual '/'join([...]) string building
• os.makedirs(exist_ok=True) replaces the if not exists guard
• f-strings replace all "token " + x + ":" + y concatenation
• requests.post/put/get replaces requests.request("POST", ...) calls
• Bare except: replaced with except Exception: throughout
• Type hints added on all public functions

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test_erpnext_sync.py + conftest.py — New test suite

Unit tests (47, always run)

Class	Coverage
TestSafeParseDt	Valid dates, None, garbage, missing microseconds
TestSafeGetErrorStr	exc extraction, plain JSON, fallback
TestResolvePunchDirection	Fixed IN/OUT, AUTO detection, unknown value
TestApplyFnToKey	Value transform, in-place mutation
TestGetDumpPath	IP sanitisation, file extension
TestGetLastLine	Missing/empty file, small/large file, multi-line
TestFindResumeIndex	No history, resume after match, import start date
TestBuildAllowlistedErrors	All errors default, subset by index
TestSendToErpnext	200 success, error response, correct URL for v13/v14
TestSendShiftSyncToErpnext	Success, failure, connection exception
TestGetShiftTypeDeviceMapping	Parsing, empty response, non-200
TestPullProcessAndPushData	Full send, resume index, allowlisted/non-allowlisted errors
TestUpdateShiftLastSyncTimestamp	Sync, skip conditions, min timestamp, backward compat

Live integration tests (9, opt-in via --live)

Hit a real ERPNext server using credentials from local_config.py.
All checkins created during the run are automatically deleted on teardown.

pytest test_erpnext_sync.py -v              # unit tests only
pytest test_erpnext_sync.py -v --live       # unit + live tests
pytest test_erpnext_sync.py -v -k Live      # live tests only

Class	What it tests
TestLiveErpNextConnection	Server reachable, credentials valid, doctype accessible
TestLiveSendToErpnext	Single IN/OUT checkin, duplicate error, unknown employee error
TestLiveBatchUpload	Multi-day mock attendance batch pushed end-to-end

Set LIVE_TEST_EMPLOYEE_ID at the top of the test file to pin a specific
employee, or leave as None to auto-detect the first active employee.

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How to test

# Unit tests (no server needed)
pytest test_erpnext_sync.py -v

# Full suite against a running ERPNext instance
pytest test_erpnext_sync.py -v --live