Problem

PR #1428 adds new behavior (SessionId enrichment, mandatory activity sink, OTel trace correlation) but has no tests covering these changes.

What Needs Testing

1. SessionId enrichment — Verify that sub-agent log events include a SessionId property matching the parent session when spawned within a session context.
2. Discard channel behavior — Verify that when activitySink is null, a discard channel is created and passed to the sub-agent without errors.
3. OTel trace correlation — Verify that the spawner's Activity has the correct parent trace ID set.
4. Tool START events — Verify that tool execution logs include a tool start event (separate from the existing result event).
5. Tool denial logging — Verify that tool denials appear at INFO level (previously DEBUG).

Proposed Tests

• Unit test: SubAgentSpawnerTests — SessionIdEnrichment_WhenSpawned_InSessionContext
• Unit test: SubAgentSpawnerTests — ActivitySink_IsDiscardChannel_WhenNotProvided
• Integration test: SubAgentActorTests — ToolStartEvents_Emitted_WithCallId
• Integration test: SubAgentActorTests — SummaryLog_IncludesCumulativeStats
• Integration test: SubAgentSpawnerTests — ToolDenials_LogAtInfoLevel

References

• PR fix(subagents): session-correlated sub-agent log observability #1428 (parent)
• SubAgentSpawner.cs
• SubAgentActor.cs