fix the session-list scan's line count, and pin both new paths with tests

The rewritten cst_agent_store_info() counted a transcript line that ran past a
whole 64 KiB read block a second time, when the newline ending its tail
arrived: a session with one very long line (a tool that read a file back)
reported one message too many -- exactly what the sidebar shows the person.

- src/agent_store.c: the tail's newline ends a line that was already counted.
- tests/test_store.c: test_info_scan() holds the scan down -- a 70 KiB line is
  one line, the first user message comes from the head of the file, and a user
  line past the window is not called the first one.
- tests/test_tools_ext.c: a byte budget stops the session_search walk, says so
  in the result, and never reports a match it did not reach; the same query
  without a budget finds it.

make check and make asan are green.

src/agent_store.c

@@ -469,7 +469,10 @@ int cst_agent_store_info(const char *workspace, const char *id,
       if (!nl)
         break;
       const size_t len = (size_t)(nl - (buf + at));
-      if (len > 0)
+      /* A line counts once. A line that ran past a whole block was counted
+       * when its tail was dropped, so the newline that ends it is not a line
+       * of its own. */
+      if (len > 0 && !slice)
         info->messages++;
       if (!slice && len > 0 && want_first && seen < kFirstUserBytes) {
         *nl = 0; /* the parse reads a C string; the newline is ours */