fix(telegram): preserve topic context for final replies

[bogdanovich]

Summary

Preserve the inbound message context when publishing normal final agent replies. This keeps Telegram forum topic metadata attached to the outbound message, so replies to topic messages are delivered back to the same topic instead of falling back to the default thread.

This is separate from the message tool path: explicit message tool sends can already carry their own context, but the regular final assistant response was still rebuilt with only channel and chat_id.

Changes

• Add an internal final-response publisher that can receive the original inbound context.
• Use it for synchronous final replies and steering final replies.
• Keep the existing public PublishResponseIfNeeded wrapper for callers that do not have inbound context.
• Add a regression test for Telegram topic final replies.

Validation

• go test ./pkg/agent -run TestProcessMessageSync_PreservesInboundTopicOnFinalResponse -count=1
• go test ./pkg/agent