Document null pointer handling in CodeRabbit AI brain implementation

[coderabbitai]

Add documentation about null pointer handling in the CodeRabbit AI brain implementation.

Key scenarios to document:

1. malloc failure in workbench initialization
2. strdup failure when copying request string

Example scenarios:

// Scenario 1: malloc fails
CodeWorkbench *workbench = (CodeWorkbench *)malloc(sizeof(CodeWorkbench));
// If malloc fails, workbench is NULL
workbench->code_request = strdup(request);  // 💥 Crash! Dereferencing NULL

// Scenario 2: strdup fails
CodeWorkbench *workbench = (CodeWorkbench *)malloc(sizeof(CodeWorkbench));
workbench->code_request = strdup(request);  // strdup returns NULL if out of memory
// Later in code:
printf("%s", workbench->code_request);  // 💥 Crash! Dereferencing NULL

Related discussion: //pull/4#discussion_r1912492452