Fix GCC 15+ build warnings on Ubuntu 26.04

[Copilot]

GCC ≥ 15 (Ubuntu 26.04) emits warnings building Citus due to stricter const-correctness on strrchr/bsearch and the new -Wunterminated-string-initialization check.

Description

Resolves four compiler warnings without behavioral change:

• -Wdiscarded-qualifiers on strrchr (cdc_decoder_utils.c, worker_data_fetch_protocol.c): store the result in a const char *. The pointer is only read, incremented, and passed to strtoull/strtou64 (first arg is const char *).
• -Wdiscarded-qualifiers on bsearch (citus_safe_lib.c): explicitly cast the return to void *. Plain cast chosen over unconstify(), whose StaticAssert fails on older compilers where bsearch returns non-const void *.
• -Wunterminated-string-initialization (multi_copy.c): mark BinarySignature[11] (intentionally non-NUL-terminated, 11 bytes) with pg_attribute_nonstring.
• Portability (pg_version_compat.h): add a pg_attribute_nonstring fallback, since the macro only exists in PostgreSQL 18+; guarded by __has_attribute(nonstring) so it degrades to a no-op where unsupported.

static const char BinarySignature[11] pg_attribute_nonstring = "PGCOPY\n\377\r\n\0";

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (efa65fc) to head (a5ec1e3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8604      +/-   ##
==========================================
- Coverage   88.73%   88.73%   -0.01%     
==========================================
  Files         288      288              
  Lines       64384    64385       +1     
  Branches     8108     8109       +1     
==========================================
- Hits        57133    57130       -3     
- Misses       4909     4913       +4     
  Partials     2342     2342              

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