Fix router stream log field accumulation

[nhynes]

Summary

• Keep per-SSE-event protocol fields on the stream log record instead of recording them back onto the long-lived router exchange span.
• Add a regression test for runtime JSON span output so stream results do not overwrite or accumulate exchange-level RPC fields.

Root Cause

SSE stream events reused the normal binding log path, which records each event's protocol summary onto the exchange span. Runtime JSON logging includes the current span fields on each event, so every stream event could add another copy of fields such as amber_protocol, amber_rpc_kind, amber_request_key, and amber_rpc_id.

Validation

• cargo fmt
• cargo test -p amber-router sse_events_do_not_record_per_event_protocol_fields_on_exchange_span
• cargo clippy --all-targets
• cargo test --workspace --all-features
• git diff --check

Closes #113