fix(tcp): unregister ActivityConnectionObserver on connection dispose to prevent memory leak

[wy471x]

Per-connection observers were registered on the shared EventBus but never
unregistered, causing stale observers to accumulate. Added onDispose cleanup
and error handling for failed downstream connections.

Per-connection observers were registered on the shared EventBus but never unregistered, causing stale observers to accumulate. Added onDispose cleanup and error handling for failed downstream connections.

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

Summary of Changes

Problem

Every inbound TCP connection in TcpBootstrapServer.bridgeConnections() created a new
ActivityConnectionObserver and registered it on the shared EventBus, but never unregistered it.
This caused:

1. Memory leak — dead observers accumulated in the EventBus, each holding stale Connection
   references in their cache map
2. Fan-out waste — each removeCommonUpstream() event was posted to every observer ever created,
   not just active ones
3. Orphaned connections — when the downstream client connection failed, the serverConn was
   never disposed and no error was logged

Fix (TcpBootstrapServer.java)

Two changes in bridgeConnections():

1. serverConn.onDispose(() -> eventBus.unregister(connectionObserver)) — ensures the observer
   is unregistered from the EventBus when the connection terminates
2. Error handler on client.subscribe() — on failure, immediately unregisters the observer,
   disposes the server connection, and logs the error

Tests (TcpBootstrapServerTest.java — new file)

4 unit tests covering the lifecycle:

Test: shouldUnregisterObserverWhenServerConnectionIsDisposed
What it verifies: Observer is unregistered when the server connection disposes
────────────────────────────────────────
Test: shouldUnregisterObserverAndDisposeServerConnOnClientConnectionError
What it verifies: On client connection failure, observer is unregistered and serverConn is
disposed
────────────────────────────────────────
Test: shouldBridgeConnectionsOnSuccessfulClientConnection
What it verifies: On success, bridge.bridge() is called with correct connections
────────────────────────────────────────
Test: shouldUseSameObserverForEventBusAndConnectionContext
What it verifies: The same observer instance is used for both EventBus registration and
connection observation

close #6448