fix(adk): reject nil events from custom agents

[yiran-culture]

What type of PR is this?

fix

Check the PR title.

• This PR title matches the format: (optional scope):
• The description is user-oriented and clear.
• No user documentation update is required; this only replaces a recovered panic with a descriptive error for invalid custom-agent output.

(Optional) Translate the PR title into Chinese.

fix(adk): 拒绝自定义 Agent 返回的 nil 事件

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

A custom Agent can currently send a nil event through its AsyncIterator. Both flowAgent.run and typedFlowAgent.run dereference that event while assigning RunPath, which triggers a nil-pointer panic. Although the outer recovery keeps the process alive, callers receive a generic panic error and stack trace instead of an actionable contract error.

This change validates events at the shared flow boundary for both Message and AgenticMessage paths. A nil event now terminates that run with a descriptive error containing the agent name. It also adds regression coverage for a nil event consumed directly through TypedRunner and through an AgentTool.

Validation:

• go test ./adk/...
• focused tests under go test -race
• git diff --check

zh:

自定义 Agent 通过 AsyncIterator 返回 nil event 时，统一 flow 管线此前会在设置 RunPath 时发生空指针 panic。该修改在 Message 和 AgenticMessage 的公共边界进行校验，将 panic 转换为包含 Agent 名称的明确错误，并补充 Runner 与 AgentTool 回归测试。

(Optional) Which issue(s) this PR fixes:

Fixes #1244

(optional) The PR that updates user documentation:

Not required.

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