fix: defer subscribe until connection_ack in GraphQLIndexer

[Abayomi-fx]

Overview

This PR fixes the GraphQL handshake race condition in GraphQLIndexer.subscribe. Previously, on socket.onopen, the client sent connection_init and then immediately sent the subscribe message in the same tick. Spec-compliant GraphQL transports reject this with 4401/4408, causing subscriptions to fail and tear down silently.

The fix introduces handshake state tracking: subscribe payloads are queued until the server sends connection_ack, then flushed immediately so the subscription is established correctly.

Related Issue

Closes the reported bug: GraphQLIndexer.subscribe sends subscribe before connection_ack.

Changes

🔌 Handshake-aware subscribe queue

• [MODIFY] src/indexer.ts
  • Add a connectionAcked handshake flag and a pending-subscribe queue to the GraphQLIndexer lifecycle.
  • When subscribe is called before connection_ack, store the parsed subscribe payload instead of sending it immediately.
  • On connection_ack, flush any queued subscribe message in the original order.
  • If the socket closes or errors before connection_ack, leave the handshake unacked and do not send the queued subscribe.
  • Preserve connection_init as the first message on open; no other protocol message is emitted until ack.

Verification Results

npm test
✅ 12/12 passed

Live acceptance check:
✅ `subscribe` not observed before `connection_ack`
✅ Queued `subscribe` delivered immediately after ack
✅ Spec-compliant ack/no-ack handshake sequences handled correctly
✅ Existing subscription flow remains functional

Acceptance Criteria	Status
subscribe is never sent before connection_ack	✅ Handshake state gates all subscribe writes
Pending subscribe payload is delivered after handshake	✅ connection_ack triggers queue flush
Compliant GraphQL servers no longer close with 4401/4408	✅ Verified with ack-delayed mock handshake
Subscriptions survive the handshake on lenient and strict servers	✅ No teardown after ack; existing flow intact

Closes #513

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