feat(adk): definition for agent_middleware

[hi-pender]

What type of PR is this?

Check the PR title.

• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

(Optional) Translate the PR title into Chinese.

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

en:
zh(optional):

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

(optional) The PR that updates user documentation:

[github-actions]

📊 Coverage Report:

File coverage threshold (20%) satisfied:	PASS
Package coverage threshold (30%) satisfied:	PASS
Total coverage threshold (83%) satisfied:	PASS
Total test coverage: 83.0% (6393/7703)