GH-48608: [Python] Fix interpolate actual values in Message.__repr__ f-string (#48656)

### Rationale for this change
`Message.__repr__` showed format placeholders (`{self.type}`) instead of actual values.

### What changes are included in this PR?
Add missing `f` prefix to f-string in `Message.__repr__` (pyarrow/lib.pxi).

### Are these changes tested?
Yes: added test verifying string representation shows actual values.

### Are there any user-facing changes?
Yes (bug fix): `str(message)` now shows values like `type: RecordBatch` instead of placeholders.
related to issue #48608
* GitHub Issue: #48608

Authored-by: aokizy <[email protected]>
Signed-off-by: Rok Mihevc <[email protected]>

Author: Aokizy2

python/pyarrow/ipc.pxi

@@ -479,7 +479,7 @@ cdef class Message(_Weakrefable):
         body = self.body
         body_len = 0 if body is None else body.size
 
-        return """pyarrow.Message
+        return f"""pyarrow.Message
 type: {self.type}
 metadata length: {metadata_len}
 body length: {body_len}"""

python/pyarrow/tests/test_ipc.py

@@ -779,6 +779,18 @@ def test_message_serialize_read_message(example_messages):
         pa.ipc.read_message(reader)
 
 
+def test_message_repr_shows_actual_values(example_messages):
+    _, messages = example_messages
+
+    for msg in messages:
+        s = str(msg)
+
+        assert '{self.type}' not in s
+        assert '{metadata_len}' not in s
+        assert '{body_len}' not in s
+        assert f'type: {msg.type}' in s
+
+
 @pytest.mark.gzip
 def test_message_read_from_compressed(example_messages):
     # Part of ARROW-5910