Allow injecting a custom ChannelManager into Device

## Summary
`ChannelManager.on_l2cap_connection_parameter_update_request` (`bumble/l2cap.py:2524-2558`) unconditionally auto-accepts every L2CAP Connection Parameter Update Request the peripheral sends, and immediately follows up with `HCI_LE_Connection_Update_Command` using the peer's requested parameters. There is no way for application code to veto the request or to substitute its own parameters.
## Motivation
I act as a GATT central talking to peripherals that, mid-session, send CPURs asking for a slower connection interval / higher latency. That kills throughput during bulk transfers I control from the central side. Today my only options are:
- Monkey-patch `ChannelManager` on the live `Device`.
- Fork bumble.
Both are ugly. I'd like a first-class way to keep the connection parameters I negotiated at connect time.
## Why I can't work around it cleanly today
- `Device.__init__` hard-codes the manager (`bumble/device.py:2449`):
```python
self.l2cap_channel_manager = l2cap.ChannelManager(
config.l2cap_extended_features
)
```
so I can't pass in a subclass.
- The existing `Connection.EVENT_CONNECTION_PARAMETERS_UPDATE` event fires *after* the update has already been issued, so it's too late to refuse.
## Proposal
Make the `ChannelManager` class injectable on `Device`:
```python
device = Device.with_hci(
"my-central",
Address(dongle_address),
source, sink,
channel_manager_cls=MyChannelManager, # default: l2cap.ChannelManager
)
```
with the matching `channel_manager_cls` kwarg threaded through `Device.__init__` and used at `device.py:2449` instead of the hard-coded `l2cap.ChannelManager`. Default behavior is unchanged.
This is the right fix because it lets advanced users override *any* L2CAP signaling handler (not just CPUR) without bumble needing to grow a new flag or event every time someone hits a hard-coded policy decision.
## Example usage
```python
class MyChannelManager(l2cap.ChannelManager):
def __init__(self, *args, **kwargs) -> None:
super().__init__(*args, **kwargs)
self.ignore_conn_param_update_req = False

def on_l2cap_connection_parameter_update_request(
self,
connection: Connection,
cid: int,
request: L2CAP_Connection_Parameter_Update_Request,
) -> None:
if not self.ignore_conn_param_update_req:
# Just bounce the request back as default accept
return super().on_l2cap_connection_parameter_update_request(
connection, cid, request
)
self.send_control_frame(
connection,
cid,
L2CAP_Connection_Parameter_Update_Response(
identifier=request.identifier,
result=L2CAP_CONNECTION_PARAMETERS_REJECTED_RESULT,
),
)

device = Device.with_hci(
"my-central", Address(dongle_address), source, sink,
channel_manager_cls=MyChannelManager,
)
```
## Environment
- bumble `0.0.230`
- Python 3.13
- Role: GATT central, LE only

Start at Device.with_hci and Device.__init__ in bumble/device.py, then inspect ChannelManager and on_l2cap_connection_parameter_update_request in bumble/l2cap.py. Trace how constructor options reach the manager, and verify that a supplied subclass is instantiated while the default behavior remains unchanged.

Written by the indexing model from the issue text.