Re: gen_guc_tables.pl: Validate required GUC fields before code generation

The following review has been posted through the commitfest application:
make installcheck-world:  not tested
Implements feature:       not tested
Spec compliant:           not tested
Documentation:            not tested

I tested patch v2 on top of current master.

- The patch applies cleanly.
- Full build and regression tests pass (“All 231 tests passed.”).
- I tested the validation by removing the `max` field from an int GUC
  (archive_timeout). The script now reports a clear error pointing to the
  exact line and missing field, instead of Perl “uninitialized value” warnings.
- Verified that the generated guc_tables.c output is unchanged.
- The added validation code is small and straightforward, and integrates
  cleanly into the existing script.

Everything works as described.  
I think this patch is ready for committer.

Reviewed-by: Mahmoud Ayman

The new status of this patch is: Ready for Committer