feat(internal/librarian/java): support alternate license header files

[sofisl]

Added support for custom external alternate license header files during Java code regeneration.
When alternate_headers is configured with a relative file path in the library's java configuration block, Librarian reads that file and appends its contents as the license header instead of the standard header. A temporary, backwards-compatible migration check ensures monolithic libraries retain their existing skip behavior until their config is updated.

There will be two f/u PRs: the first in google-cloud-java to add the property and the header. The second in librarian to remove the monolithic block since (I think) it only applies to grafeas, and just rely on the alternate headers path.

For #6178

[gemini-code-assist]

Code Review

This pull request introduces support for alternate license headers in Java modules by adding an AlternateHeaders configuration field and updating the post-processing logic to read from this file if provided. The review feedback focuses on improving test robustness by using bytes.HasPrefix instead of bytes.Contains to verify headers, optimizing the license text retrieval to return []byte directly to avoid unnecessary string conversions, and properly wrapping errors.

[sofisl]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for configuring alternate license headers in Java modules. It adds an alternate_headers field to the configuration schema and the JavaModule struct, and updates the post-processing logic to read and apply the alternate header text from the specified file path. Comprehensive unit tests have been added to verify this new behavior, including error handling for missing header files. No review comments were provided, so there is no additional feedback to address.