Add scenario 9 manual test for nested MarkdownJsonExtractor

[ahaoli]

Motivation

• Add a targeted case to validate recursive extraction of the innermost JSON when a json fence appears inside another json fence (input like result: ```json\n```json {"a":1, "b":"hello"} ```\n``` done).

Description

• Added a manual test invocation in MarkdownJsonExtractor.main that calls run("场景9 - 多层嵌套（递归提取最内层）", "result: ```json\n```json {\"a\":1, \"b\":\"hello\"} ```\n``` done"); to assert the extractor returns the innermost JSON content.

Testing

• Attempted to run mvn -q compile, but automated compilation failed due to dependency/plugin resolution error from Maven Central (403 Forbidden), so no further automated tests were executed in this environment.

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Codex Task