feat: schema mapping, delete fix, and gap closures CEP-6

[harsh04044]

Final PR closing CEP-6 implementation #76.

What's in this PR:

Schema-to-kind mapping table - replaced the field-presence if-else chain in handle_announcement_response() with a typed mapping table. Each entry validates field types (arrays must be arrays, strings must be strings), matching TS SDK's safeParse semantics. Rejects malformed payloads like {"tools": null}.

Delete fix - delete_announcements() now fetches existing events per kind via fetch_events() and publishes kind 5 with ["e", event_id] tags for each found event. Added fetch_events() to RelayPoolTrait and MockRelayPool.

Integration tests - roundtrip publish→discover for all 5 kinds, delete tag format verification, schema rejection tests.

Gap closures - 5 tests added for CEP-6 parity with TS SDK:

• profile metadata custom field preservation through kind 0 content
• profile metadata error resilience (no panic on publish failure)
• private server publishes relay list but not announcements
• relay_list_urls and bootstrap_relay_urls are independent
• profile metadata published regardless of is_announced_server

Also fixed a wrong integration test that expected the old ["k", kind] deletion behavior.