Feat: Add system-wide stream handler unregister method.

Summary

py-libp2p supports registering protocol stream handlers via set_stream_handler, but does not provide a guaranteed, system-wide way to unregister them.

This creates inconsistent service lifecycle behavior across modules.

Motivation

We need lifecycle symmetry for stream protocols in existing service lifecycle methods:

• service startup path should register handler(s)
• service shutdown path should unregister handler(s).

Without a standard unregister API, services currently use module-specific workarounds, which makes behavior inconsistent and harder to reason about on long-running hosts.

Current state

Core interfaces currently expose registration but not guaranteed removal:

• IHost.set_stream_handler(...):
  link
• IMultiselectMuxer.add_handler(...):
  link

No required remove_stream_handler / remove_handler contract exists at those interface levels.

Existing workaround patterns

1. Flags-only stop (service marked stopped, handler still registered)

   • Bitswap stop path:
     link

2. Empty handler replacement as pseudo-unregister

   • DCUtR sets an empty_handler:
     link

3. Optional unregister with fallback if method is missing

   • Relay protocol tries remove_stream_handler, falls back on AttributeError:
     link

Proposed change (stream scope only)

1. Add interface methods:

• IHost.remove_stream_handler(protocol_id: TProtocol) -> None
• IMultiselectMuxer.remove_handler(protocol: TProtocol) -> None

2. Implement in core:

• BasicHost.remove_stream_handler(...) delegates to multiselect
• Multiselect.remove_handler(...) removes the protocol from handler map

3. Service migration (follow-up in same or separate PRs):

• Update services that register handlers to unregister on stop/teardown
  (perf, bitswap, kad_dht, relay/DCUtR, etc.)

Expected impact

• Consistent register/unregister semantics for stream protocols during service startup/shutdown
• Fewer ad-hoc unregister workarounds
• Cleaner behavior for long-lived hosts and protocol restarts

Are you planning to do it yourself in a pull request ?

yes

Start with the IHost and IMultiselectMuxer contracts in libp2p/abc.py, then locate BasicHost and Multiselect implementations. Confirm that BasicHost delegates removal and Multiselect removes the protocol from its handler map, using the listed bitswap, DCUtR, relay, perf, and kad_dht lifecycle paths as migration context. Done means the interfaces and core implementations provide the removal API with consistent stream-handler teardown semantics.