Fix throughput-spike OTA timeout build by defining work item before use

[MacNite]

Motivation

• The spike variant failed to build because the spike-only helper ota_release_arm_timeout() referenced arm_timeout_work before that delayable work item was defined, which only occurs when ENABLE_THROUGHPUT_SPIKE is enabled.

Description

• Adjusted firmware-nrf54lm20a/src/ota.c so the reboot/stall handler declarations and the K_WORK_DELAYABLE_DEFINE for arm_timeout_work, reboot_work, and stall_timeout_work appear before the #if ENABLE_THROUGHPUT_SPIKE block, allowing ota_release_arm_timeout() to reference arm_timeout_work without compile errors.
• This is strictly a source-order fix and does not change runtime behavior for non-spike builds.

Testing

• git diff --check completed with no new warnings or errors.
• A source-order assertion (Python check) verified the K_WORK_DELAYABLE_DEFINE for arm_timeout_work now precedes the ota_release_arm_timeout() helper.
• west build for the spike variant was not executed here because the Nordic/Zephyr sysbuild toolchain is not available in this environment, so a full rebuild remains recommended on a machine with the SDK installed.

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Codex Task