[Python] RecordBatch.serialize() silently drops dictionary messages

### Describe the bug, including details regarding any error messages, version, and platform.

Hi Arrow Friends,

`RecordBatch.serialize()` on a batch with dictionary encoded columns produces a single IPC record batch message containing only the indices. The dictionary values message is not included. Attempting to read the result back with `ipc.read_record_batch()` fails with `ArrowKeyError: Dictionary field not found.`

A full IPC stream via `ipc.new_stream` correctly writes schema, dictionary, and record batch messages and round-trips fine.

This is surprising because serialize() succeeds without error, producing bytes that cannot be deserialized.

I think we have some gaps in the PyArrow IPC API regarding `DictionaryMemo` and the ability to interpret `Dictionary` IPC messages.

To see the issue:

```python
import pyarrow as pa
from pyarrow import ipc

arr = pa.array(["apple", "banana", "apple", "cherry", "banana"]).dictionary_encode()
batch = pa.record_batch([arr], names=["fruit"])

# serialize() produces only a record batch message — no dictionary message
raw = batch.serialize().to_pybytes()

reader = pa.BufferReader(pa.py_buffer(raw))
while (msg := ipc.read_message(reader)) is not None:
print(msg.type)
# Output: record batch

# Round-trip fails
ipc.read_record_batch(pa.py_buffer(raw), batch.schema)
# ArrowKeyError: Dictionary field not found
```

For comparison, a full IPC stream includes the dictionary:

```python
sink = pa.BufferOutputStream()
writer = ipc.new_stream(sink, batch.schema)
writer.write_batch(batch)
writer.close()
stream = sink.getvalue().to_pybytes()

reader = pa.BufferReader(pa.py_buffer(stream))
while (msg := ipc.read_message(reader)) is not None:
print(msg.type)
# Output: schema, dictionary, record batch
```

This bug is kind of a motivating/organizing one. I'm going to be creating a series of PRs that increase the PyArrow API around dictionary messages so that it is easier for users to serialize IPC RecordBatches with dictionaries.

Rusty

### Component(s)

Python