fix(python/adbc_driver_manager): reimport PyArrow if installed mid-session

[amoeba]

Currently, with the dbapi module in the Python driver manager, if I am using a REPL or notebook session started without PyArrow installed, I have to restart my session to pick PyArrow up so I can use PyArrow-enabled methods. This is because the module attempts to import PyArrow at startup and sets a persistent flag that never changes after import.

It would be nicer if we could rescue the user so they can install PyArrow in their environment while a REPL or notebook is running, make the call again, and have it work.

Steps to reproduce:

1. Create an environment without PyArrow:

python3 -m venv .venv
source .venv/bin/activate
pip install adbc_driver_manager

2. See we get the expected error when we try to use a PyArrow-enabled routine:

.venv $ python
Python 3.14.0 (main, Oct 14 2025, 21:10:22) [Clang 20.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from adbc_driver_manager import dbapi
>>> con = dbapi.connect("sqlite://:memory:")
>>> cur = con.cursor()
>>> cur.execute("SELECT 1").fetch_arrow_table()
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    cur.execute("SELECT 1").fetch_arrow_table()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/bryce/src/apache/arrow-adbc/test_env/.venv/lib/python3.14/site-packages/adbc_driver_manager/dbapi.py", line 1355, in fetch_arrow_table
    return self._results.fetch_arrow_table()
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/bryce/src/apache/arrow-adbc/test_env/.venv/lib/python3.14/site-packages/adbc_driver_manager/dbapi.py", line 1532, in fetch_arrow_table
    return _blocking_call(self.reader.read_all, (), {}, self._stmt.cancel)
                          ^^^^^^^^^^^
  File "/Users/bryce/src/apache/arrow-adbc/test_env/.venv/lib/python3.14/site-packages/adbc_driver_manager/dbapi.py", line 1466, in reader
    _requires_pyarrow()
    ~~~~~~~~~~~~~~~~~^^
  File "/Users/bryce/src/apache/arrow-adbc/test_env/.venv/lib/python3.14/site-packages/adbc_driver_manager/dbapi.py", line 1588, in _requires_pyarrow
    raise ProgrammingError(
    ...<2 lines>...
    )
adbc_driver_manager.ProgrammingError: This API requires PyArrow to be installed

3. Try to fix it in another shell

source .venv/bin/activate
pip install pyarrow

4. Run (2) again and get the same error

The Python driver manager's dbapi module guards PyArrow-enabled routines with _requires_pyarrow(). This PR tweaks that helper and lets it attempt to re-import PyArrow but only if PyArrow wasn't enabled at import time.

This PR was generated using an LLM.