`UserProfileFunctions.cs`: Deserialization errors return generic 400 with no structured logging

[AB-Law]

Problem

When request body deserialization fails in UserProfileFunctions, the handler returns a generic BadRequest("Invalid request body.") with no log entry:

File: PluckIt.Functions/Functions/UserProfileFunctions.cs:63

catch (JsonException)
{
    return new BadRequestObjectResult("Invalid request body.");
}

Operators have no way to see what payload caused the failure, which field was missing, or how often this happens.

Proposed Fix

Add structured logging before returning:

catch (JsonException ex)
{
    logger.LogWarning(ex, "Profile deserialization failed for request to {Path}", req.Path);
    return new BadRequestObjectResult("Invalid request body.");
}

Functionality Impact

None. Same 400 response to callers, but now logged for operators.