fix: surface deepest diagnostic error event

[snkk2x-collab]

Summary

• Track diagnostic event call depth from fn_call / fn_return topics while enriching reports.
• Select the deepest diagnostic event that carries an error payload and add it as a high-likelihood root cause.
• Cover nested diagnostic streams with helper-level and XDR-backed enrich_report tests.

Validation

• rustfmt --check crates/core/src/decode/diagnostic.rs
• cargo test -p prism-core --lib

Closes #245

[codeZe-us]

Hi @snkk2x-collab please you were not assigned to work on this task. Apply to the task on Grantfox and i will assign