Serialize sensor measurements with BLE sessions

[MacNite]

Motivation

• Prevent audio sessions from starting inside a long-running sensor measurement cycle that can reconfigure/stop the PDM and power off the microphone rail.
• Ensure the sensor rail is not silently turned off by a different caller when another user enabled it, avoiding latent ownership bugs and mid-session power loss.

Description

• Add a session gate semaphore session_gate and APIs link_measurement_begin() / link_measurement_end() to serialise an entire powered measurement cycle against BLE sessions so a session either starts before a cycle or waits for it to finish.
• Keep GATT callbacks non-blocking by making link_claim() non-blocking (returns busy when the gate is unavailable) and transferring long work to session threads; move audio rail enable + mic_stream_start() out of the GATT write callback into the audio capture thread.
• Make the sensor rail refcounted and mutex-protected (sensor_rail_users with sensor_rail_lock) so each successful power_sensor_rail_enable() increments an owner count and the hardware is disabled only when the last matching power_sensor_rail_disable() runs.
• Update documentation/comments: clarify mic.h to state that the session gate guarantees exclusivity between the one-shot measurement path and streaming PDM capture, and ensure the main measurement loop now holds the session gate for the full cycle.

Testing

• git diff --check and repository integrity checks were executed and reported no issues.
• The Zephyr/west --sysbuild images were not built because west was unavailable in the execution environment, so the three configured build profiles and the firmware size delta could not be measured.
• Hardware behavioural validation (repeated 30-second recordings including START while a measurement cycle is running) was not run here and remains required to confirm the microphone-power and PDM exclusivity fixes on target hardware.

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Codex Task