fix: avoid trace executor panics

[umeeSthein]

Summary

• Replace production unwrap() calls in the trace executor with error handling.
• Return default tracer serialization failures through trace/RPC errors instead of panicking.
• Preserve partial block trace results when mux inspector recreation fails.

Tests

• cargo fmt --all --check
• cargo check -p debug-trace-server --message-format=short
• cargo test -p debug-trace-server tracing_executor --message-format=short