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.

tests/test_tools_ext.c

@@ -236,6 +236,50 @@ static void test_session_search(cst_tool_ctx_t *ctx) {
   c = mk("session_search", NULL);
   EXPECT(run(ctx, c, &r) == 1, "a query is required");
   cst_tool_result_free(&r);
+
+  /* A store is not read to the end on a whim: a byte budget stops the walk
+   * and says so, and it never reports a match it did not get to. */
+  {
+    char path2[512];
+    EXPECT(cst_agent_store_path_for_id(g_ws, "20260101-130000-big", path2,
+                                       sizeof(path2)) == 0,
+           "a second session path");
+    cst_session_t *s2 = cst_session_open(path2);
+    EXPECT(s2 != NULL, "its transcript opens");
+    if (s2) {
+      cst_message_t m2;
+      memset(&m2, 0, sizeof(m2));
+      m2.role = CST_ROLE_USER;
+      m2.content = (char *)"the start of a long one";
+      EXPECT(cst_session_append(s2, &m2) == 0, "a first line goes in");
+      for (int i = 0; i < 600; i++) {
+        memset(&m2, 0, sizeof(m2));
+        m2.role = CST_ROLE_ASSISTANT;
+        m2.content = (char *)"a line of tool output that says nothing special";
+        if (cst_session_append(s2, &m2) != 0)
+          break;
+      }
+      memset(&m2, 0, sizeof(m2));
+      m2.role = CST_ROLE_ASSISTANT;
+      m2.content = (char *)"the needle is only at the very end";
+      EXPECT(cst_session_append(s2, &m2) == 0, "the needle goes in last");
+      cst_session_close(s2);
+    }
+    /* Enough lines that a byte budget is spent before the end is reached. */
+    c = mk("session_search", "query", "needle", "max_bytes", "4096", NULL);
+    EXPECT(run(ctx, c, &r) == 0 && r.out != NULL, "a bounded search runs");
+    EXPECT(r.out && strstr(r.out, "budget stopped the walk") != NULL,
+           "the byte budget stops the walk and says so");
+    EXPECT(r.out == NULL || strstr(r.out, "the needle is only") == NULL,
+           "and it does not report a match it never got to");
+    cst_tool_result_free(&r);
+    /* The same query without a budget reaches the end and finds it. */
+    c = mk("session_search", "query", "needle", NULL);
+    EXPECT(run(ctx, c, &r) == 0 && r.out &&
+               strstr(r.out, "the needle is only") != NULL,
+           "the unbounded search finds it");
+    cst_tool_result_free(&r);
+  }
 }
 
 /* --- device_info: the environment, without a shell ---------------------- */