Enhancement: Fix Kademlia DHT Bootstrap by Using Connected Peers as Fallback

[SuchitraSwain]

issue: #905

### What was wrong?

• After bootstrap, peers were added to the peerstore but not yet in the routing table.
• DHT queries called find_local_closest_peers(), which returned an empty list.
• This blocked random walks, provider lookups, and peer discovery.
• As a result, nodes remained isolated and couldn’t progress beyond initial bootstrap peers.

### How was it fixed?
A fallback mechanism was implemented in the DHT query logic:

• Configurable threshold: Uses connected peers from the peerstore when the routing table has fewer than MIN_PEERS_THRESHOLD = 5 entries.
• Distance-based sorting: Connected peers are sorted by XOR distance to the target key.
• Integration points: Added across peer_routing.py and kad_dht.py for FIND_NODE, GET_VALUE, PUT_VALUE, and provider lookups.
• Graceful handling: Falls back only when necessary; no impact when routing table is sufficiently populated.
• Tech requirements / highlights
• No breaking changes to existing DHT APIs.
• Threshold is configurable for different network environments.
• Fully covered with 3 new unit tests ensuring fallback correctness.
• All existing peer routing and DHT tests updated and passing (28/28 total).

✅ Benefits

• Eliminates bootstrap deadlock.
• Ensures peer discovery works immediately after startup.
• Improves resilience and connectivity in sparse networks.
• Maintains Kademlia’s routing efficiency by preferring routing table peers when available.

To-Do

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

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