[P4][Logs] Standardize structured field names and remove duplicate keys

Parent issue: #4691

Intent

Field naming is currently inconsistent across modules, and some logs even reuse the same key for different values. That makes aggregation and search harder than necessary. This task standardizes field names and removes duplicate-key cases so log platforms can query on stable dimensions.

Modification points

• Define consistent field names for core identifiers such as changefeed, dispatcher, node, subscription, region, event service, operator, and mode.
• Remove duplicate keys inside a single log entry and align equivalent concepts to the same key name across modules.
• Prefer structured field consistency over free-text parsing in message bodies.

Entry points

• maintainer/operator/operator_controller.go:268
• server/server.go:422
• downstreamadapter/dispatcherorchestrator/dispatcher_orchestrator.go:247

Done criteria

• Equivalent identifiers use the same field names across the targeted paths.
• No targeted log entry contains duplicate keys with ambiguous overwrite behavior.
• Aggregation and filtering can rely on structured fields rather than message parsing.

Review the targeted log paths in maintainer/operator/operator_controller.go:268, server/server.go:422, and downstreamadapter/dispatcherorchestrator/dispatcher_orchestrator.go:247. Compare structured fields for equivalent identifiers and identify duplicate keys within each log entry. Done means consistent names across the targeted paths, no ambiguous duplicate keys, and fields usable for aggregation without parsing message text.