feat(core): await connect-time requests before subscribe

[MaxSiominDev]

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Pull request overview

This PR introduces a connection-readiness barrier so callers can block until the initial IPROTO_ID exchange and connect-time watcher subscriptions are acknowledged, preventing early usage from racing with connect-time requests.

Changes:

• Add a registered() completion signal to WatcherStateMachine that completes on the first successful watch response (or exceptionally on error/kill/send failure).
• Add IProtoClient.awaitConnectionReady() and implement it in IProtoClientImpl by joining on server protocol negotiation + watcher registration futures.
• Add unit/integration tests validating watcher registration completion and that awaitConnectionReady() does not hang.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tarantool-core/src/main/java/io/tarantool/core/protocol/fsm/WatcherStateMachine.java	Tracks watcher “registered” completion and fails it on errors/kill/send failure.
tarantool-core/src/main/java/io/tarantool/core/IProtoClientImpl.java	Implements awaitConnectionReady() by waiting for connect-time async futures to complete.
tarantool-core/src/main/java/io/tarantool/core/IProtoClient.java	Adds new blocking API awaitConnectionReady().
tarantool-core/src/test/java/io/tarantool/core/protocol/fsm/WatcherStateMachineTest.java	New unit tests for watcher registration completion (note: currently uses Java 9+ APIs).
tarantool-core/src/test/java/io/tarantool/core/integration/IProtoClientWatchersTest.java	Integration test ensuring awaitConnectionReady() completes promptly.

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