pw_rpc2: Improve call lifecycle, handshake, and cancellation

- Call lifecycle & status tracking: Track completion with an explicit
  flag bit rather than a sentinel status, allowing Status::Unknown()
  to represent unrecognized peer error codes without ambiguity.
- Cancellation: Make Cancel() complete calls locally with CANCELLED so
  pending reads and responses resolve immediately. Add Reader::Cancel()
  and cancel client calls when uncompleted readers or futures drop.
- Server call retirement: Send a terminal error packet to the peer if a
  server call is retired while its writer is still open, preventing
  clients from hanging indefinitely on abandoned methods.
- Client teardown: Track live Client handles with a refcount and
  automatically close the connection when the last handle is dropped.
  Streamline ClientConnectionTask closure using an atomic state and a
  binary semaphore turnstile.
- Handshake negotiation: Transmit the negotiated version in handshake
  packets, accept packets of 8 or more bytes for forward compatibility,
  and reject version 0.
- Sizing & write reservations: Add PwpbSerializer::ReservationSize to
  size bounded messages by exact content when the schema maximum exceeds
  transport MTU. Reject reservation commits on closed/cancelled calls.
- Listener robustness: Re-arm Accept() and re-enqueue after connection
  allocation failures to prevent listeners from stalling.
- Header organization: Split connection_task.h into connection_task.h,
  client_connection_task.h, and server_connection_task.h to match the
  source file structure.

Change-Id: I5dac22f509a6a50b8e023cdad14d21a0a9deed26
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sandbox/+/489809