fix(keyence): read full barcode reply and release the read beam (LOFF)

[dinukajayalath]

Fixes #1101

Problem:

KeyenceBarcodeScannerBackend.scan_barcode() returns only the first character of a barcode, and leaves the reader's beam latched on:

• send_command() calls self.io.read() with no num_bytes, and Serial.read defaults to 1 byte — no loop, no terminator scan so
  The reader's \r-terminated reply is truncated to one character.
• scan_barcode() sends LON (which latches the read beam on) but never the matching LOFF, so the beam stays on after the call.

Verified on hardware (Keyence BL-600HA, Code 128 label Test): PLR returned data='T'; the same reader driven directly (LON →read to \r → LOFF) returned Test.

Fix:

• send_command accumulates bytes until \r, stopping at the terminator or at the first byte-less read (the port timeout=1
  elapsed with no reply — e.g. no barcode present, a valid empty result). Replies now come back whole, and a present barcode returns the instant the terminator lands instead of blocking on a fixed byte count.
• scan_barcode sends LOFF in a finally block to release the beam, whether the read succeeded or raised. LOFF runs through a try/except that logs rather than propagates, so a LOFF failure can't mask the scan's result or error.

Tests

keyence_backend_tests.py. Each fails against pre-fix code and passes after:

• send_command: accumulates to \r, stops at the terminator, treats a byte-less read as an empty reply, writes \r-terminated
  commands.
• scan_barcode: sends LOFF after LON on success / NG / ERR99; a failing LOFF masks neither a successful result nor a reader error.

Glad to make any change upon reviewing.