Omit redundant Dex flow type headers

[longquanzheng]

Summary

• omit the inherited Dex flow type header when an activity has a FlowTypeProvider
• treat the activity provider as authoritative and normalize empty values to none
• retain inherited headers for subflow and system activities without a flow type provider
• verify the persisted activity history and metrics behavior

Rationale

Step and RPC activity inputs already contain their flow type. Persisting the same value in __temporal_sdk_dex_flow_type adds redundant bytes to every scheduled activity event.

User impact

New histories omit the private header for activities with a flow type provider. Existing histories and activities without a provider remain compatible. Metric names and non-empty labels are unchanged.

Validation

• go run . check
• go run . unit-test -run 'TestDex'
• go test -count 1 -race ./replaytests -run 'TestReplayTestSuite/TestReplayLegacyActivityHeaderWithDexFlowTypeProvider'
• SERVICE_ADDR=127.0.0.1:7233 SERVICE_HTTP_ADDR=127.0.0.1:7243 go run . integration-test -run 'TestIntegrationSuite/TestDex'
• SERVICE_ADDR=127.0.0.1:7233 SERVICE_HTTP_ADDR=127.0.0.1:7243 WORKFLOW_CACHE_SIZE=0 go run . integration-test -run 'TestIntegrationSuite/TestDex'