test(networking): add gossipsub handler behavioral test vectors

[tcoratger]

Summary

Introduces a new GossipsubHandlerTest fixture that tests gossipsub protocol decisions, not wire encoding. This is a fundamentally different kind of test vector — it captures what the node DOES when it receives GRAFT/PRUNE/IHAVE/IWANT/messages, not what bytes it produces.

Each vector defines:

• Initial state: mesh topology, peer subscriptions, backoff timers, seen/message caches
• Incoming event: an RPC from a specific peer
• Expected output: outbound RPCs sent + resulting mesh topology

Why this matters

If two clients disagree on whether to accept a GRAFT, they have incompatible mesh topologies. If they disagree on IHAVE filtering, lazy pull breaks. These are the most interop-critical decisions after wire format.

Vectors (14 total)

Handler	Count	What's tested
GRAFT	5	Accept, reject (capacity at d_high), reject (backoff), ignore (unsubscribed), idempotent
PRUNE	2	Remove from mesh + set backoff, zero backoff
IHAVE	3	Unseen triggers IWANT, seen produces no IWANT, mixed filtering
IWANT	1	Cached message returned as full message
Message	3	Forward to mesh except sender, duplicate not forwarded, IDONTWANT peer skipped

Design decisions

• No randomness: handler code paths don't use random.sample() — fully deterministic
• Time control: single now timestamp, backoffs are absolute values, time.time() is patched
• Peer names: simple Base58-valid strings ("peerAx", "senderX"), converted internally
• New fixture class: separate from NetworkingCodecTest because the semantics are different (state+event→behavior vs input→encoding)

Test plan

• uvx tox -e all-checks passes
• uv run fill --fork=devnet --clean -n auto -- tests/consensus/devnet/networking/ generates all 107 fixtures
• Verified GRAFT accept adds peer to mesh with no RPCs sent
• Verified GRAFT reject (backoff) sends PRUNE with 60s backoff
• Verified IHAVE for unseen message triggers IWANT with exact message ID
• Verified message forwarding skips sender and IDONTWANT peers

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