fix: Preserve streams resources when migrating workspace aliases

[tcannon91]

Summary

• Prevents stream connection replacement when an unchanged workspace is migrated from instance_name to workspace_name.
• Makes the alias migration one-way: reverting a resource that already uses workspace_name back to instance_name is rejected during planning.
• Avoids unnecessary stop/update/restart calls for alias-only stream processor changes and documents the migration.

Testing

• go test ./internal/service/streamconnection ./internal/service/streamprocessor
• git diff --check

Notes

• Acceptance coverage was added but not run locally because it requires configured Atlas acceptance-test credentials and infrastructure.