fix: fix cli not reading stdin

[UziTech]

Marked version: 18.0.3

Markdown flavor: CLI

Description

• Fixes CLI Does not Work in 18.0.3 #3965

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Project	Deployment	Actions	Updated (UTC)
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[gemini-code-assist]

Code Review

This pull request refactors the CLI input handling and enhances the test suite with integration tests. The review identifies a compatibility risk with import.meta.dirname for Node.js versions prior to 20.11.0 and suggests reverting to the previous __dirname approach. Additionally, it notes that shell interpolation for stdin in tests is fragile and recommends using child_process.spawn for improved robustness.