fix: conform kad-dht interop node to the cross-implementation contract

[adust09]

The kad-dht interop node added in #264 was only ever exercised against itself.
Running it against the actual libp2p/unified-testing matrix (py, dotnet, nim)
turned up two defects. Both are in interop/kad-dht-node/Main.hs; the library
is untouched.

1. Value payload did not follow the suite's convention

The provider stored <TEST_KEY>-value and the querier compared for exact
equality. Every other implementation stores hello from <impl> client and
matches on the hello from substring:

impl	stores	matches
py	hello from py client	b"hello from" in value
dotnet	hello from dotnet client	strVal.Contains("hello from")
haskell (before)	<TEST_KEY>-value	exact equality

py's node carries the comment "Accept any 'hello from' message for
cross-language interop", so the substring match is the intended contract.

The DHT itself was working the whole time — a py querier found the haskell
provider and retrieved its record, then rejected the payload:

provider (haskell) | putValue succeeded
querier  (py)      | Test 2 -> Success! Found 1 provider(s)!
querier  (py)      | Test Failed: Expected value containing 'hello from',
                   |              but got b'702199ac-value'

Now stores hello from haskell client and matches on the shared marker.

2. Identify handlers were never registered

No role called registerIdentifyHandlers. dotnet-libp2p awaits
/ipfs/id/1.0.0 on every outbound connection before its dial resolves —
Libp2pPeerFactory.ConnectedTo calls session.DialAsync<IdentifyProtocol>(),
and Peer.DialAsyncCore blocks on session.Connected until that returns — so
a node that does not serve it is undialable from dotnet:

provider (dotnet) | Ctx(1): Session created by dialer for /ip4/.../p2p/12D3KooWFW11...
provider (dotnet) | Unhandled exception. System.TimeoutException: The operation has timed out.
                  |    at Nethermind.Libp2p.Core.LocalPeer.DialAsyncDeduped(...)

This was directional: haskell dialing dotnet worked, dotnet dialing haskell
failed 100% of the time (7/7 cases). Registered in all three roles, since DHT
peers dial each other in every role, not just the bootstrap.

Results

Full local run of the 37 unified-testing matrix tests that involve haskell
(bootstrap x provider x querier over py, dotnet, nim, haskell):

	passed
#264 as merged	8 / 37
+ value convention	30 / 37
+ Identify handlers	37 / 37

For reference, the same harness scores 24/27 on the pre-existing py/dotnet/nim
matrix (three dotnet-querier tests fail independently of this change).