fix(processor): preserve op_idx in host event errors

[KitaroMoha563]

Describe your changes

Partially addresses #1693.

Preserve the emitting operation index when host event handling fails in the fast processor.

This threads op_idx through the host event diagnostic path for both event handler failures and advice mutation failures raised while applying host-returned mutations. It also adds targeted diagnostic tests to verify both branches now highlight the emit.event(...) instruction instead of the first operation in the basic block.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Commits are signed.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.
• Updated `CHANGELOG.md'

[github-actions]

Automated check (CONTRIBUTING.md)

Findings:

• Ensure issue Improve processor error messages #1693 is assigned to the PR author.
• Add a short Rationale explaining why the change is needed.

Recommendations:

• Consider adding a Test plan or clear review steps.

Next steps:

• Ensure issue Improve processor error messages #1693 is assigned to you.
• See CONTRIBUTING.md for expectations.
• If this is a false positive, comment: /quality-review.