[Internal] Add per-resource unified provider telemetry to User-Agent for SDKv2

[tanmay-db]

Changes

• Adds unifiedprovider/true to the User-Agent header when a user sets provider_config.workspace_id in their Terraform configuration for a SDKv2 resource or data source
• Uses useragent.InContext (per-request context), so only API calls from resources where the user has configured provider_config include the tag — all other resources are
  unaffected

Tests

• TestAddUnifiedProviderContext_WithProviderConfig — unifiedprovider/true present when user sets workspace_id in their config
• TestAddUnifiedProviderContext_WithoutProviderConfig — absent when user does not set provider_config
• TestAddUnifiedProviderContext_WithEmptyWorkspaceID — absent when user sets workspace_id to empty string
• All existing context and unified provider tests pass

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/terraform

Inputs:

• PR number: 5461
• Commit SHA: 3e284dd92d7265e78a2aab7c8993578f21c81a4a

Checks will be approved automatically on success.