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How heap buffer overflow happens in C UART response handling and how to fix it

A critical heap buffer overflow vulnerability was discovered in the AT client response handler (`sm_at_client.c`) where incoming UART data was copied into a fixed-size buffer without verifying available capacity. A compromised modem or malicious UART data could trigger arbitrary heap corruption. The fix replaces an assertion-only guard with proper bounds clamping using `MIN()` to ensure writes never exceed the `at_cmd_resp` buffer allocation.