[flutter_tools] Support heartbeat / liveness monitoring for long-running tool extension RPCs #193955

In #193748, ExtensionBuildManager.build invokes the build.build RPC on a tool extension isolate without a wall-clock timeout (timeout: null in ExtensionConnection.sendRequest) because complex native builds can legitimately run for many minutes.

While ExtensionConnection already monitors Isolate.addOnExitListener and Isolate.addErrorListener to fail in-flight requests immediately if an extension isolate crashes or exits, a silently deadlocked extension isolate during a long-running operation (build.build, artifact.downloadArtifacts, etc.) will hang indefinitely without a liveness mechanism.

We should consider introducing a heartbeat / progress notification mechanism between package:flutter_tools_extension and ExtensionConnection:

1. Allow long-running extension RPC handlers (or the extension isolate runner) to emit periodic heartbeat or build progress notifications over the JSON-RPC channel.
2. Support an optional inactivity/heartbeat timeout on ExtensionConnection.sendRequest that resets whenever a heartbeat or progress notification is received from the extension, rather than bounding total wall-clock execution time.