Fix printing long strings in %whos magic command. (ipython#14938)

In trying to improve printing for dataframe and series objects, in pull
request ipython#14926, i inadvertently messed up printing of long
strings...sorry!

This change fixes that issue.

```python
## Does not mess up long strings
In [1]: sunday_qry = """
   ...: with
   ...:     df as (select unnest(generate_series(date '1900-01-01',date '2100-12-31',interval 1 day)) as dt)
   ...:   select * from df where extract(dayofweek from dt)=0;
   ...: """
   ...:

In [2]: df = pd.DataFrame({"a": range(10), "b": range(10,20),"c":list(string.ascii_letters[:10])})
In [3]: %whos
Variable     Type         Data/Info
-----------------------------------
df           DataFrame    Shape: (10, 3)
sunday_qry   str          \nwith\n    df as (select<...>t(dayofweek from dt)=0;\n
In [4]:
"""
```

Compare this to what it currently does.
```python
## inadvertently messed up in ipython#14926 
In [1]: sunday_qry = """
   ...: with
   ...:     df as (select unnest(generate_series(date '1900-01-01',date '2100-12-31',interval 1 day)) as dt)
   ...:   select * from df where extract(dayofweek from dt)=0;
   ...: """
   ...:

In [2]: df = pd.DataFrame({"a": range(10), "b": range(10,20),"c":list(string.ascii_letters[:10])})
In [3]: %whos
Variable      Type             Data/Info
----------------------------------------
df            DataFrame        Shape: (10, 3)
sunday_qry    str
with
    df as (select unnest(generate_series(date '1900-01-01',date '2100-12-31',interval 1 day)) as dt)
  select * from df where extract(dayofweek from dt)=0;

In [4]:
```

Author: krassowski

IPython/core/magics/namespace.py

@@ -470,9 +470,9 @@ def type_name(v):
                 # Useful for DataFrames and Series
                 # Ought to work for both pandas and polars
                 print(f"Shape: {var.shape}")
-            elif hasattr(var, "__len__"):
+            elif hasattr(var, "__len__") and not isinstance(var, str):
                 ## types that can be used in len function
-                print(var if isinstance(var, str) else f"n={len(var)}")
+                print(f"n={len(var)}")
             else:
                 try:
                     vstr = str(var)

tests/test_magic.py

@@ -732,6 +732,40 @@ def __repr__(self):
     _ip.run_line_magic("whos", "")
 
 
+@pytest.mark.parametrize(
+    "input,expected",
+    (
+        (
+            "The quick brown fox jumps over the lazy dog",
+            "The quick brown fox jumps over the lazy dog",
+        ),
+        (
+            " ".join(["The quick brown fox jumps over the lazy dog"] * 2),
+            "The quick brown fox jumps<...>x jumps over the lazy dog",
+        ),
+        ("\nThis is \n\na long\n\nstring\n", r"\nThis is \n\na long\n\nstring\n"),
+        (
+            "\rThis is \r\ra long\r\rstring\r",
+            r"\rThis is \r\ra long\r\rstring\r",
+        ),
+    ),
+)
+def test_whos_longstr(input, expected):
+    ip = get_ipython()
+    ip.user_ns["input"] = input
+    ip.user_ns["expected"] = expected
+    with capture_output() as captured:
+        ip.run_line_magic("whos", "")
+    stdout = captured.stdout
+    from_whos = " ".join(
+        re.split(
+            r"\s+str\s+",
+            [l for l in stdout.splitlines() if l.startswith("expected")][0],
+        )[1:]
+    )
+    assert expected == from_whos
+
+
 def doctest_precision():
     """doctest for %precision