Fix C bugs, add audit comments, and deduplicate FK check code

Bug fixes:
- Fix DatumNegativeInfinity passing text* to numeric_in (expects cstring).
  Use CStringGetDatum + DirectFunctionCall3 instead of cstring_to_text.
- Fix write_history not validating SPI_fnumber result before SPI_gettypeid,
  matching the validation pattern in generated_always_as_row_start_end.

Audit comment:
- Add detailed comment block above FK/UK plan cache declarations documenting
  lifecycle, transaction scoping, cleanup callback, and session-scoped plans.

Refactor:
- Extract shared fk_check_c() helper from fk_insert_check_c/fk_update_check_c,
  eliminating ~250 lines of near-identical code. The only difference between
  insert and update is which tuple to validate (tg_trigtuple vs tg_newtuple).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jhf

src/covers_without_gaps.c

@@ -414,9 +414,10 @@ Datum DatumNegativeInfinity(Oid elem_oid)
             return DateADTGetDatum(DATEVAL_NOBEGIN);
         case NUMERICOID:
         {
-            text* negativeInfinityText = cstring_to_text("-Infinity");
-            Datum negativeInfinityDatum = DirectFunctionCall1(numeric_in, PointerGetDatum(negativeInfinityText));
-            pfree(negativeInfinityText);
+            Datum negativeInfinityDatum = DirectFunctionCall3(numeric_in,
+                CStringGetDatum("-Infinity"),
+                ObjectIdGetDatum(InvalidOid),
+                Int32GetDatum(-1));
             return negativeInfinityDatum;
         }
         case TIMESTAMPOID: