chore(librarian): Only add header to files in .librarian/generator-input

[parthea]

This PR fixes a bug introduced in #14901 which adds the following text to all generated files. We only want this text added to .librarian/generator-input

# DO NOT EDIT THIS FILE OUTSIDE OF `.librarian/generator-input`
# The source of truth for this file is `.librarian/generator-input`

[gemini-code-assist]

Summary of Changes

Hello @parthea, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a bug introduced in a previous change that caused a specific 'DO NOT EDIT' header to be erroneously applied to all generated files. The fix refines the file processing workflow to ensure that this header is now correctly and exclusively added to files residing in the .librarian/generator-input directory, aligning with the intended behavior and preventing unintended modifications to other generated outputs.

Highlights

• Bug Fix: Corrected an issue where a 'DO NOT EDIT' header was being added to all generated files, instead of exclusively to files within the .librarian/generator-input directory.
• File Processing Logic: Implemented a temporary directory approach during file copying to isolate the header application process, ensuring the header is only applied to the intended files before they are moved to their final destination.

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[gemini-code-assist]

Code Review

This pull request addresses a bug where a header was incorrectly added to all generated files. The proposed solution correctly isolates the intended files within a temporary directory, applies the header, and then moves them to the final destination. This approach is clean, effective, and resolves the issue described. The change is well-implemented and improves the generator's behavior.