tests/bsdiff: add content_size guard boundary test

tests/bsdiff: add content_size guard boundary test

Add test_bspatch_content_size_guard to verify the integer truncation
guard added for RHEL-189207.  The test exercises the exact boundary
condition used in dispatch_bspatch():

  content_size > G_MAXSIZE || content_size > (guint64)G_MAXINT64

It validates that:
- Values with bit 63 set and G_MAXUINT64 are always rejected
- On 32-bit systems, values above G_MAXUINT32 (4 GiB) are rejected
  including the PoC value 0x100001000 (4 GiB + 4 KiB)
- Normal values (0, 1, 4096, 1 GiB) are accepted and the casts to
  gsize and int64_t produce correct values