Etherlink/Tezt: register validate testsuite for both mainnet and latest kernels

What

This MR extends the Etherlink validation test suite to run tests against both mainnet and latest EVM kernels, instead of only testing with the latest EVM kernel.

Why

Previously, the validation tests were only running against the latest EVM kernel (Constant.WASM.evm_kernel). To ensure compatibility and prevent regressions across different kernel versions, we need to validate that our test suite works correctly with both the mainnet kernel and the latest development kernel.

How

The changes modify the register function in etherlink/tezt/tests/validate.ml to:

1. Add an optional kernels parameter that defaults to both Constant.WASM.mainnet_kernel and Constant.WASM.evm_kernel
2. Wrap the existing test registration logic in an additional loop that iterates over all specified kernels
3. Update test titles and tags to include kernel-specific information using Uses.tag kernel
4. Pass the kernel parameter to the init_sequencer_sandbox function
5. Update the uses field in test registration to use the current kernel instead of hardcoding Constant.WASM.evm_kernel

This approach maintains backward compatibility while expanding test coverage. Each test case now generates separate test instances for each kernel, with clear naming to distinguish between them.

Manually testing the MR

1. Run the validation test suite: dune exec etherlink/tezt/main.exe -- --file etherlink/tezt/tests/validate.ml
2. Verify that tests are now running for both kernel variants (check test output for kernel-specific tags in test names)
3. Confirm that all existing test functionality works as expected for both mainnet and latest kernels
4. Check that test results are properly tagged and distinguishable between kernel versions

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR