refactor: address PR review feedback on tests, bridge error handling, and schema consistency

[Copilot]

Addresses all actionable review comments from the initial test PR: silent error discard, weak round-trip assertions, list-format YAML bug, schema/implementation mismatch, brittle type assertions, and cross-file mock coupling.

Changes

• bridge.go: Propagate MetaWalkMut error instead of discarding with _

  if err := msg.MetaWalkMut(func(key string, value any) error {
      meta[key] = value
      return nil
  }); err != nil {
      return nil, fmt.Errorf("copy message metadata: %w", err)
  }

• bridge_test.go: Round-trip test now asserts body fields and metadata key/value are preserved, not just that the result is non-nil

• processor_step_test.go: Fixed context-cancellation test — was using YAML list format (- bloblang: ...) which AddProcessorYAML rejects, so the test never exercised cancellation; switched to single-object format and uses errors.Is for the wrapped error

• plugin.go: Removed Required: true from processors schema field — the constructor accepts nil as pass-through, so the schema was incorrectly signaling the field is mandatory

• plugin_test.go: Replaced all NewBentoPlugin().(*bentoPlugin) type assertions with SDK interface assertions (sdk.ModuleProvider, sdk.StepProvider, sdk.TriggerProvider, sdk.SchemaProvider); panics on future implementation changes instead of silently passing

• mocks_test.go (new): mockPublisher / mockSubscriber extracted from input_module_test.go into a dedicated shared file, eliminating implicit compilation dependencies across test files

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