feat(extensibility): migrate n8n hook execution to per-workflow MCP tool invocation

[rishikunnath2747]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Migrates n8n hook execution in the extensibility module from a workflow-ID-based polling model (using instance mcp) to a per-workflow MCP tool invocation model.

What changed:

• N8nWorkflowConfig dataclass: workflow_id field removed; replaced with ord_id, card_ord_id, tool_name, and global_tenant_id
• UMS GraphQL fragment updated to fetch the four new fields instead of workflowId
• _build_hook() now skips hooks missing either toolName or cardOrdId (previously guarded on workflowId)
• _discover_n8n_tools() now filters discovered MCP tools by both tool_name and card_ord_id (each hook is its own MCP server)
• _execute_workflow_via_agw() rewritten: single call_mcp_tool() call; response is expected to be an A2A Message returned directly (no polling, no execution-tree traversal)
• _poll_hook_execution() deleted — polling is no longer part of the AGW execution path
• call_hook() (legacy direct-HTTP path) is unchanged
• local_extensibility_example.json and user-guide.md updated to reflect the new schema
• emit_extensions_summary_span now includes joule_studio_gsid sourced from ext_impl.joule_studio_gsid, propagating the Joule Studio global solution ID to the aggregate summary span

Type of Change

• New feature (non-breaking change that adds functionality)

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Additional Notes

The call_hook() method (direct HTTP path via HookConfig) is intentionally left unchanged. Only call_hook_agw() and the underlying AGW helper methods are modified by this PR.