[P4][Logs] Keep a deep-diagnostics mode while making default logs compact

Parent issue: #4691

Intent

Reducing default log volume must not remove the ability to debug complex online issues. This task preserves a deep-diagnostics mode so detailed logs remain available when needed, while keeping the default path concise and operationally useful.

Modification points

• Add or reuse a diagnostics switch, local log-level override, or dynamic level policy for targeted components.
• Keep the default path on compact summaries and essential fields.
• Route heavy object dumps and per-object traces to diagnostics-only logging.

Entry points

• logservice/txnutil/lock_resolver.go:56
• downstreamadapter/eventcollector/dispatcher_stat.go:670
• logservice/logpuller/subscription_client.go:1055

Done criteria

• Default logs remain compact after the reduction work.
• Teams can still enable targeted deep diagnostics without restoring global log noise.
• The separation between default operational logs and diagnostics-only logs is explicit and documented.

Start by reading the logging paths at logservice/txnutil/lock_resolver.go:56, downstreamadapter/eventcollector/dispatcher_stat.go:670, and logservice/logpuller/subscription_client.go:1055. Trace how default summaries, heavy dumps, and per-object traces are emitted, then verify that default logs stay compact, targeted deep diagnostics remain available, and the separation is explicitly documented.