Implement server-side pipeline handlers with CEL intention validation

[philbrookes]

Description

Implement the server-side PipelineOnRequest and PipelineOnResponse handlers in internal/extension/manager.go.

Key behaviours:

• Validate each action in the batch (action type, predicates, method references)
• For grpc_method actions: validate Intention CEL against the response proto schema using Phase 2 ProtoCache
• If any action fails validation, reject the entire batch (no partial writes)
• Append all actions to PipelineActionStore with sequential indices
• Trigger reconciliation

Acceptance Criteria

• PipelineOnRequest handler implemented
• PipelineOnResponse handler implemented
• CEL intention validation against proto schema for grpc_method actions
• Atomic batch validation — all-or-nothing
• Unit tests
• Integration tests

References

• Design: extensions-SDK-action-pipeline-design.md
• Parent: Extension SDK Action Pipeline #1898