[v10] Make traceId required on SentryLog #3835

Context

Part of the Major v10 Task Collection.

Problem

SentryLog.traceId is nullable and defaults to SentryId.empty(), so logs can ship without a real trace association. There is an existing TODO(major-v10).

Expected outcome

• traceId becomes non-nullable and a required constructor parameter.
• The SentryId.empty() fallback is removed.

Pointers

• packages/dart/lib/src/telemetry/log/log.dart:7-27

Breaking change: Compile-time for direct SentryLog constructors (mostly internal callers).