Fix three build and link breaks in shared code

[mmmorks]

Summary

Three one-line fixes of the same kind — code that does not compile or link somewhere — kept together because splitting them further would be three trivial PRs.

ConfigSerializer.cpp does not compile in the host test build. It calls atoi/atol/atof and reaches them through Arduino.h on MCU targets, but not in the native googletest environment. On macOS the whole suite fails to compile at ConfigSerializer.o, taking every unit test down with it. Include <stdlib.h>.

LocationProvider::sendSentence() is declared but never defined. It was the class's key function under the Itanium ABI, so the base vtable is un-emittable and any translation unit that emits it gets an undefined reference. Every current subclass overrides it, which is why no build has failed yet. Give it an empty default body, matching the no-op override in EnvironmentSensorManager.cpp. This is a latent hazard, not the host-test-build failure above — the native env compiles neither this header nor its users.

CustomLLCC68Wrapper::doResetAGC() does not compile. It calls sx126xResetAGC() with one argument; the only overload takes (SX126x*, bool rx_boost_gain). No environment in the tree compiles this wrapper yet, but the first board to use it will not build. Pass getRxBoostedGainMode(), as the SX1262, SX1268 and STM32WLx wrappers do.

How it was tested

• pio test -e native: all suites pass on macOS (before the <stdlib.h> fix none of them compile there).
• Compiled for meshnology_w12_repeater (ESP32-S3).

Dependencies

Independent. Applies to dev.

The same <stdlib.h> include is also carried in a PR against the l5yth/meshcore-linux fork, which needs it for its own host build; whichever lands first makes the other's hunk a no-op.