[roll] Roll fuchsia [usb-virtual-bus] Refactor state management and connection logic

Redesigns the virtual bus connection state machine to explicitly track
connecting and disconnecting phases. This ensures that FIDL Connect and
Disconnect requests correctly synchronize between the virtual DCI
(peripheral) and HCI (host) components.

Key changes:
- Implements a 4-state machine (Disconnected, Connecting, Connected,
  Disconnecting) and tracks hardware-level DCI/HCI connectivity flags.
- Ensures Connect() operations sequentially activate DCI then HCI,
  while Disconnect() deactivates HCI then DCI.
- Handles overlapping or redundant FIDL requests by queuing callbacks
  and returning appropriate statuses (ZX_OK or ZX_ERR_BAD_STATE).
- Updates StartController and StopController (DCI) to trigger state
  transitions in the background, avoiding deadlocks with the peripheral
  driver during initialization and teardown.
- Enhances error handling and rollback if a phase of connection or
  disconnection fails.
- Improves FIDL documentation for the Bus protocol to clarify error
  behaviors and state transitions.

Adds comprehensive unit tests to verify:
- Overlapping Connect/Disconnect requests.
- Error propagation and rollback during Hci/Dci failures.
- Idempotent Connect/Disconnect calls.
- Banjo control and data request flows during transitions.

Original-Bug: 497086698
Test: usb-virtual-bus-unit-test
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1557222
Original-Revision: 0110145d398a1a5b7e95a96b86aced8b69917345
GitOrigin-RevId: 2321b8119511e7f7a760a0c3368af14530bcca9c
Change-Id: I91e52d64b55ab62246a039bdfb01708604beb475