feat(mesh): add BLE advertisement protobuf enums

[Komzpa]

What does this PR do?

Adds protobuf enum values needed by the firmware BLE advertisement bearer scaffold:

• Config.NetworkConfig.ProtocolFlags.BLE_ADVERTISEMENT_BROADCAST for enabling BLE advertisement packet broadcasts.
• MeshPacket.TransportMechanism.TRANSPORT_BLE_ADVERTISEMENT so packets received through that bearer can be tagged distinctly.
• Removes one pre-existing blank line that buf format rejects in meshtastic/mesh.proto, so this PR can pass the current format gate after touching that file.

This supports meshtastic/firmware#10292; that firmware draft should stay draft until this schema lands and generated bindings are refreshed from the canonical protobufs submodule.

Related Issue

Validation

• buf format --diff --exit-code --path meshtastic/config.proto --path meshtastic/mesh.proto .
• buf lint --path meshtastic/config.proto --path meshtastic/mesh.proto .
• buf lint .
• protoc -I. --include_imports --descriptor_set_out=/tmp/meshtastic-ble-adv.desc meshtastic/*.proto nanopb.proto
• git diff --check origin/master...HEAD
• codex review --base origin/master

Checklist before merging

• All top level messages commented
• All enum members have unique descriptions