feat: enhance webhook error messages with execution guidance

Replace generic "Please try again later or contact support" error messages
with actionable guidance that directs users to use n8n_get_execution with
mode='preview' for efficient debugging.

## Changes

### Core Functionality
- Add formatExecutionError() to create execution-specific error messages
- Add formatNoExecutionError() for cases without execution context
- Update handleTriggerWebhookWorkflow to extract execution/workflow IDs from errors
- Modify getUserFriendlyErrorMessage to avoid generic SERVER_ERROR message

### Type Updates
- Add executionId and workflowId optional fields to McpToolResponse
- Add errorHandling optional field to ToolDocumentation.full

### Error Message Format

**With Execution ID:**
"Workflow {workflowId} execution {executionId} failed. Use n8n_get_execution({id: '{executionId}', mode: 'preview'}) to investigate the error."

**Without Execution ID:**
"Workflow failed to execute. Use n8n_list_executions to find recent executions, then n8n_get_execution with mode='preview' to investigate."

### Testing
- Add comprehensive tests in tests/unit/utils/n8n-errors.test.ts (20 tests)
- Add 10 new tests for handleTriggerWebhookWorkflow in handlers-n8n-manager.test.ts
- Update existing health check test to expect new error message format
- All tests passing (52 total tests)

### Documentation
- Update n8n-trigger-webhook-workflow tool documentation with errorHandling section
- Document why mode='preview' is recommended (fast, efficient, safe)
- Add example error responses and investigation workflow

## Why mode='preview'?
- Fast: <50ms response time
- Efficient: ~500 tokens (vs 50K+ for full mode)
- Safe: No timeout or token limit risks
- Informative: Shows structure, counts, and error details

## Breaking Changes
None - backward compatible improvement to error messages only.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/types/n8n-api.ts

@@ -290,6 +290,8 @@ export interface McpToolResponse {
   message?: string;
   code?: string;
   details?: Record<string, unknown>;
+  executionId?: string;
+  workflowId?: string;
 }
 
 // Execution Filtering Types