Processes a string containing a function call description and returns the
result of evaluating that function call. The function call description
is in the format:
@@function:object#id@@
Where:
* function is the name of the function to call
* object is the object to call the function on
* id is the object's ID
The function call will be evaluated and its result returned.
Note: This function is different from parse_command() which uses OBJ, OBJ2,
PREP, and WORD tokens for pattern matching. This function is specifically
for evaluating a single function call and returning its result.
Example:
```c
mixed value = process_value("@@query_weight:/obj/weapon#456@@");
// Returns the weight of the weapon object
```