Sanitize LLDP text and tolerate malformed port IDs

[ashutosh-agrawal]

Summary

Harden LLDP synchronization against malformed remote text and port identifiers. Parse otherwise-valid lldpctl JSON containing raw control characters, strip C0/C1 controls recursively from keys and values before they reach Redis or logs, and keep unknown or malformed port-ID shapes from aborting the entire update.

Motivation

A malformed TLV should not disable LLDP synchronization for unrelated interfaces. The existing parser can also raise on an unknown or non-object port ID before its exception handler runs.

Changes

• Sanitize parsed LLDP text at the subprocess-output boundary.
• Preserve a usable port ID value when its subtype is unknown, while defaulting malformed fields safely.
• Add regression cases for control characters and malformed port-ID shapes.

Testing

The Azure sonic-dbsyncd pipeline passed at PR head 2b0160df. Its Python 3 job runs python3 setup.py test and publishes test results and coverage. A separate local attempt to run tests/test_lldpSyncDaemon.py stopped during collection because mockredis was unavailable; no local pass is claimed.

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