fix(mqtt): coarsen public position uplinks

[Komzpa]

Summary

• dual-publish decoded POSITION_APP packets to the default public MQTT server
• publish a generated coarse packet first with precision_bits = 15, then publish the original packet unchanged
• apply the same ordering to JSON MQTT output when JSON is enabled

Why

The public MQTT broker currently filters precise position packets. Publishing the coarse packet first lets current public consumers receive a location update, while publishing the original second preserves current behavior for private/custom brokers and for any future public-broker policy that accepts more precise positions.

Notes

• Only decoded POSITION_APP packets sent to the default public MQTT server are duplicated/coarsened.
• Encrypted uplinks remain unchanged.
• The generated coarse packet keeps the same mesh metadata but masks latitude/longitude to the public precision bucket.

Tests

• Added/updated test_mqtt coverage for protobuf dual publish, JSON dual publish, and encrypted-position behavior.
• Built heltec-v3 successfully with PlatformIO.

[Copilot]

Pull request overview

Coarsens public-MQTT POSITION_APP uplinks to address the public broker's filtering of precise positions. When the configured MQTT server is the default public one, decoded position packets are dual-published: first a generated copy with coordinates bucketed to ≤15 precision bits (and a fresh packet id), then the original packet unchanged. JSON output mirrors the same ordering. Encrypted protobuf uplinks are intentionally left as-is. When the offline queue has only one slot left, only the imprecise copy is queued so the public-usable variant survives eviction.

Changes:

• New getPublicMqttPositionPrecision() and makePublicMqttPositionPacket() helpers in src/mqtt/MQTT.cpp that build a coarsened POSITION_APP copy.
• MQTT::onSend refactored around a publishPacket lambda that handles a single (proto, json) publish; it's now invoked twice for public-server position packets, with a queue-pressure carve-out.
• New test_mqtt cases covering imprecise-then-original ordering for protobuf and JSON, coarser-source preservation, no-coordinates skipping, encrypted-stays-encrypted, and offline-queue-near-full behavior; the mock MQTT server's command parser was rewritten to handle multi-packet writes and varint remaining-length.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/mqtt/MQTT.cpp	Adds the public-position coarsening helpers and reworks onSend to dual-publish (coarse + original) for default-public-broker decoded position packets, with queue-pressure handling.
test/test_mqtt/MQTT.cpp	Adds position helpers, drainQueue helper, multi-packet/varint-length parsing in the mock MQTT server, JSON-topic capture, and new test cases for the dual-publish behavior.