stream_server: decode requests in the per-stream read task

[mo4islona]

Follow-up to #211, based on net-stream-server-worker so the diff shows only the delta.

Makes ServerBehaviour generic over the request message type and moves protobuf decoding from the worker actor into the per-stream read task.

Motivation:

• Undecodable requests are dropped at the source, before taking a slot in the requests queue and the worker events queue, and the warn line now names the protocol.
• Decoding runs concurrently (one task per stream), keeping protobuf parsing off the swarm event loop — with request_response it used to run in per-connection tasks.
• The worker handlers lose the three repeated decode-and-log blocks and become one-liners again; the typed QueryBehaviour/LogsBehaviour/StatusBehaviour aliases are restored.

The cost is a single type parameter on ServerBehaviour/Request — request side only: responses stay raw bytes written via ResponseSender, so there is no codec pair as in the old request_response server.

Tests now encode a proper protobuf message (TestMsg), plus a new undecodable_request_rejected case covering the decode-failure path.

Verified locally: cargo test -p sqd-network-transport --features worker --lib (12 passed), cargo clippy --workspace --all-targets --all-features -- -D clippy::correctness, cargo +nightly fmt --check.

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