Fix recording-file parameter ignores for pcap recording method

[fergusean]

Summary

When using the pcap recording method, the recording-file parameter passed in a "start recording" NG command was silently ignored — the file was always created at an auto-generated path (<spooldir>/pcaps/<callid>-<random>.pcap) instead of the caller-specified one.

Root Cause

The recording-file value is correctly parsed and stored in the call struct, but the pcap initialization code never consulted it when opening the file.

Changes

• Add recording_setup_file_explicit() to open a pcap file at a caller-specified absolute path
• Update pcap init to use the explicit path when recording-file is set, falling back to auto-generation otherwise
• Clear stored file path after recording finishes to prevent stale reuse
• Add NULL guards and fix memory leak in cleanup routines
• Fix use-after-free crash on shutdown (reorder recording_fs_free() after call_free())
• Add regression tests covering the explicit-path recording flow