refactor(whatsrust): extract event conversion

[Andiveli]

Summary

• Extract CEvent to Event conversion into events.rs.
• Preserve specialized event mappings, discriminants, field conversion, and callback registration behavior.

Changes

• Moved the event callback translator and chat, reaction, and message-action conversion helpers out of lib.rs.
• Kept the root facade API and ABI unchanged.

Chain Context

• Start: immediate parent refactor/whatsrust-incoming-messages at 13e69a4 / PR refactor(whatsrust): extract incoming message conversion #300.
• End: event conversion is owned by events.rs.
• Dependency: review after PR refactor(whatsrust): extract incoming message conversion #300.
• Diagram: PR #299 -> 📍 PR (incoming) -> 📍 PR (events)

Review Budget

• Authored diff: 248 lines (128 additions, 120 deletions).
• Budget: <=400 touched lines.

Testing

• cargo fmt --all -- --check
• git diff --check
• Source checks confirm the event handler registration remains present.
• Cargo compilation/tests: deferred explicitly to CI; not run locally.
• Manual runtime testing: N/A for this mechanical extraction.

Rollback and Out of Scope

• Rollback: revert commit 6a22a9a; only event conversion extraction is removed.
• Out of scope: behavior fixes, API redesign, ABI changes, Cargo builds/tests, and unrelated cleanup.

Contributor Checklist

• Linked issue: refactor(whatsrust): extract event conversion #297
• Exactly one type label: type:refactor
• No modified shell scripts; shellcheck not applicable
• Conventional commit format
• No Co-Authored-By trailers

Closes #297