Deprecate v2 SenderBuilder::new, fix FFI v1 panic #1148

Job	Run time
Verify pub.dev package	4m 8s
Build and test dart (macos-latest)	5m 49s
Build and test dart (ubuntu-26.04)	5m 38s
Verify release tag	0s
Publish to pub.dev (automated publishing / OIDC)	0s
	15m 35s

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