fix: improve Chrome trace event format correctness and metric naming

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Improve tracing feature.

What changes are included in this PR?

• Write memory counter values in bytes instead of MB, with _bytes suffix on the args key, so trace viewers display correct values
• Include the process ID in the trace filename (comet-event-trace-{pid}.json) so each executor writes its own file instead of corrupting a shared one
• Use the actual process ID in trace events instead of hardcoded 1
• Replace .expect() panics with graceful error handling: the writer becomes None on failure so tracing silently disables itself
• Add Drop impl to flush the BufWriter on shutdown so buffered events are not lost at process exit
• Replace fragile ThreadId debug-string parsing with ThreadId::as_u64().get() (stable since Rust 1.74)
• Rename CometUnsafeShuffleWriter trace event to snake_case comet_unsafe_shuffle_writer for consistency
• Fix "comet_shuffle_" metric name (trailing underscore, no thread ID) in CometBypassMergeSortShuffleWriter to match the pattern used in CometUnsafeShuffleWriter
• Rename jvm_heapUsed metric to jvm_heap_used (snake_case)
• Emit shuffle_spilled_bytes counter event after each shuffle spill

How are these changes tested?