fix: address low/informational sec-parser follow-ups

- client_cli: capture errno before output_escape() in
  read_patterns_from_file() so an over-long line is still reported as
  EFBIG instead of the (possibly malloc-clobbered) errno.
- file_list: guard string_list_add() capacity doubling against
  overflow (capacity > INT_MAX / 2), matching filter_rule_list_add();
  callers already surface the false as a memory-allocation error.
- compression: ZSTD_isError() is true for ZSTD_CONTENTSIZE_UNKNOWN,
  which made the 3x unknown-size fallback dead code.  Test the
  CONTENTSIZE_ERROR/UNKNOWN sentinels explicitly so unknown-size frames
  reach the estimate path (still bounded by the existing hard limit)
  while invalid frames are rejected.  Known-size frames and the 100 MB
  ceiling/overflow checks are unchanged.
- tests: add an unknown-content-size-frame decompression test.

Tests: ./build/tests and ./build-asan/tests all pass (42/42);
clang-format + cppcheck clean.

src/client/client_cli.c

@@ -2045,13 +2045,16 @@ static int read_patterns_from_file(const char* filepath, char*** patterns, int*
   while (true) {
     ssize_t n = utils_getdelim_bounded(fp, &line, &line_size, '\n', UTILS_MAX_LINE_LEN);
     if (n < 0) {
+      /* output_escape() may allocate (and clobber errno): capture the reader's
+       * errno first so an over-long line is still reported as EFBIG. */
+      int saved_errno = errno;
       char* escaped = output_escape(filepath, false);
-      if (errno == EFBIG) {
+      if (saved_errno == EFBIG) {
         log_message(LOG_LEVEL_ERROR, "pattern file '%s' has a line exceeding %d bytes",
                     escaped ? escaped : "<allocation failed>", (int)UTILS_MAX_LINE_LEN);
       } else {
         log_message(LOG_LEVEL_ERROR, "could not read pattern file '%s': %s",
-                    escaped ? escaped : "<allocation failed>", strerror(errno));
+                    escaped ? escaped : "<allocation failed>", strerror(saved_errno));
       }
       free(escaped);
       free(line);