fix(server): keep discover lifecycle bootstrap-neutral

[faisal-shah]

Summary

• keep metadata-bearing server/discover requests bootstrap-neutral
• select the classic lifecycle on a subsequent initialize
• select the inline lifecycle only on the first valid ordinary metadata-bearing request
• leave malformed and unsupported bootstrap requests recoverable
• enqueue the lifecycle-selecting inline request into the normal service loop so bidirectional handlers remain safe

Verification

• cargo test --package rmcp --features "client transport-io macros schemars"
• cargo clippy --package rmcp --features "client transport-io macros schemars" --lib --test test_server_initialization --test test_stateless_server_requests --test test_subscriptions -- -D warnings