feat: add chat stream helpers

[Cr1stal]

Adds a structured helper API for the Slack chat stream methods.

This is a cleaned-up follow-up to #1545 with a smaller review surface:

• Adds ChatStream and AsyncChatStream facades for chat.startStream, chat.appendStream, and chat.stopStream.
• Separates stream buffering, lifecycle validation, protocol calls, and async queueing into focused package-private helpers.
• Keeps the sync helper caller-confined while the async helper serializes work and protects mutable stream state.
• Adds local sync and async coverage for buffering, lifecycle failures, stop semantics, serialization, recovery, and async ordering.

User/developer impact: callers can work with Slack chat streams through a small stateful helper instead of manually coordinating method calls and stream timestamps.

Validation:

./mvnw -pl slack-api-client test -Dtest=test_locally.api.methods.ChatStreamTest,test_locally.api.methods.AsyncChatStreamTest

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[WilliamBergamin]

Closing since this is a duplicate of #1545

[Cr1stal]

@WilliamBergamin Thanks, that makes sense.

I opened this PR only because I could not push the cleaned-up/squashed version directly to slackapi:ale-chat-stream-helper. Let's keep #1545 as the canonical PR.

For reference, the cleaned branch is here:
https://github.com/Cr1stal/java-slack-sdk/tree/feature/pr-1545-local-rescue

It keeps the same intent but reduces the branch to two commits:

• Add chat stream helpers
• Add chat stream helper tests

If #1545 should remain the canonical PR, could someone with push access either update ale-chat-stream-helper from that branch or let me know the preferred way to transfer the cleaned diff there?