feat: server identity parsing, relay list fetching, and fetch_events (CEP-17)

[harsh04044]

Part of CEP-17 implementation #80.

Adds all the standalone building blocks for client-side relay discovery - no transport wiring yet, that's PR 2.

What's new:

parse_server_identity() - accepts hex pubkey, npub, or nprofile. Extracts relay hints from nprofile for use in relay resolution later. NostrClientTransport now uses this instead of PublicKey::from_hex(), so nprofile identifiers work as serverPubkey today.

fetch_server_relay_list() - creates a temporary relay pool, connects to discovery relays, fetches kind 10002, returns parsed RelayListEntry vec with url + optional marker. Disconnects in all code paths.

select_operational_relay_urls() - picks operational relays from a relay list. Prefers unmarked entries, falls back to read+write union, deduplicates.

RelayPoolTrait::fetch_events() - added Vec variant to the trait and all impls (RelayPool, MockRelayPool, TestRelayPool). MockRelayPool also gets inject_event() for test setup.

[ContextVM-org]

Overall I think it looks good to merge.
My only minor note is that RelayPool::fetch_events() currently loops over filters one by one; that’s functionally correct for the current usage, but if nostr-sdk supports batching multiple filters in one fetch call, that might be worth revisiting later for efficiency. Not a blocker for this PR.