fix(basemgr): trailing newline in FreeSWITCH Backgrounding stderr check

[brancan]

Problem

fsinstance() in basemgr.py checks FreeSWITCH stderr to detect a successful background start:

# Before
if stderr and not stderr.endswith('Backgrounding.'):
    result = False
    logger.error(...)

FreeSWITCH actually outputs Backgrounding.\n (with a trailing newline). The old check always evaluates to True on a normal startup, so every successful FreeSWITCH launch logs a false ERROR and sets result = False.

Fix

# After
if stderr and not stderr.endswith('Backgrounding.\n'):

One character change — matches the actual FreeSWITCH output.

Notes

This bug was also independently identified in PR #179. This PR isolates the fix as a standalone, minimal change with no other modifications.