fix(flows): end invocation on a deferred long-running tool call

[copybara-service]

fix(flows): end invocation on a deferred long-running tool call

A long-running tool (e.g. a human-in-the-loop request) that returns no
immediate result defers its response until the caller supplies it later.
The framework built an empty function-response for such a call and
re-invoked the model with that placeholder, so the model proceeded on an
empty result -- fabricating a completion, or in multi-tool agents
re-issuing calls in a runaway loop until the LLM call limit.

• Skip the function-response event when a long-running tool returns null or
  an empty result, so no placeholder is sent back to the model.
• Treat an event that carries pending long-running tool-call ids as a final
  response, so the invocation ends and returns control to the caller.
• Guard LlmAgent output-key writes so a text-less final event (such as the
  long-running call itself) no longer overwrites the output key with an
  empty string.

This brings Event.finalResponse() to parity with ADK Python's
Event.is_final_response, which returns true when long_running_tool_ids is
set (in both the 1.x and 2.0 branches), and the output-key guard mirrors
ADK Python's output_key handling.