feat(kernel): add Display impl for DebugEvent telemetry

[SH20RAJ]

Summary

• implement Display for DebugEvent using structured single-line output: [timestamp_ms] event_type: key=value ...
• include readable formatting for all variants (workflow_start, node_start, state_change, node_end, workflow_end, error)
• use <none> for missing old_value and <global> for error events without node_id
• add 8 focused unit tests covering all variants and edge cases requested in feat: add Display impl for DebugEvent telemetry type #988

Why

This improves readability for CLI logs, dashboards, and realtime telemetry streams while preserving the existing serialized event schema.

Testing

• Added unit tests in crates/mofa-kernel/src/workflow/telemetry.rs
• Could not execute cargo test in this environment because cargo is unavailable in PATH

Closes #988

[SH20RAJ]

Ready for review: this PR implements issue #988 with a Display formatter and 8 unit tests covering all variants + edge cases. Please let me know if you want any output style tweaks before merge.

[SH20RAJ]

Clarification: the formatter mentioned above is the Display implementation for the DebugEvent telemetry type.