Implement WriteTo trait for gix::Blob

[Copilot]

This PR implements the WriteTo trait for gix::Blob<'repo>, enabling blob objects to be used with APIs that require this trait, such as Repository::write_object().

Changes Made

The implementation adds the WriteTo trait to gix::Blob<'repo> in gix/src/object/impls.rs with the following methods:

• write_to() - writes the blob's data verbatim using write_all
• kind() - returns gix_object::Kind::Blob
• size() - returns the length of the blob's data as u64

This follows the same pattern as the existing WriteTo implementations for gix_object::Blob and gix_object::BlobRef.

Example Usage

use gix_object::WriteTo;

// Create a blob
let blob_id = repo.write_blob(b"hello world")?;
let blob = repo.find_object(blob_id)?.into_blob();

// Now you can use the blob with write_object (which requires WriteTo)
let written_id = repo.write_object(blob)?;

// Or use WriteTo methods directly
let mut buffer = Vec::new();
blob.write_to(&mut buffer)?;
assert_eq!(blob.kind(), gix_object::Kind::Blob);
assert_eq!(blob.size(), 11);

Testing

Added comprehensive tests covering:

• Basic functionality with write_object()
• All WriteTo trait methods (write_to, kind, size)
• Edge cases like empty blobs
• Integration with existing blob functionality

All existing tests continue to pass, ensuring no regressions.

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