Fix mixed filament merge, slice-all preview, and TLS cert override

[wudi901]

Description

This follow-up PR fixes three independent issues:

• Sidebar::merge_mixed_filament() now passes the remapped target ID to on_filaments_delete(), so object/volume extruder assignments follow the merged target instead of falling back to filament 1.
• Plater::priv::on_process_completed() now resets m_slice_all after slicing completes, preventing stale slice-all state from blocking the selected plate from loading correctly in Preview.
• Http::tls_system_cert_store() now checks the result of getenv() before constructing a std::string, avoiding undefined behavior when the OpenSSL certificate override environment variable is not set.

Affected files:

• src/slic3r/GUI/Plater.cpp
• src/slic3r/Utils/Http.cpp

Screenshots/Recordings/Graphs

Before/after screenshots or recordings may be attached after manual GUI verification.

Tests

• Windows Release build.
• GitHub Actions Build all.
• Slice all plates, return to Prepare, select an individual plate, and verify Preview loads that plate.
• Merge a mixed filament and verify object/volume extruder assignments use the remapped target.
• Verify the OpenSSL certificate override path when SSL_CERT_FILE is unset.