test: Add comprehensive unit tests for workflow versioning services

Add 158 unit tests (157 passing, 1 skipped) across 5 new test files to
achieve strong coverage of the workflow versioning and auto-update features.

New test files:
- workflow-versioning-service.test.ts (39 tests)
  * Version backup, restore, deletion, pruning
  * Version history and comparison
  * Storage statistics and auto-pruning
  * Edge cases: missing API, version not found, restore failures

- node-version-service.test.ts (37 tests)
  * Version discovery and caching (with TTL)
  * Version comparison and upgrade analysis
  * Breaking change detection and confidence scoring
  * Upgrade path suggestions and intermediate versions

- node-migration-service.test.ts (32 tests, 1 skipped)
  * Node parameter migrations (add/remove/rename/set default)
  * Webhook UUID generation
  * Nested property migrations
  * Batch workflow migrations with validation

- breaking-change-detector.test.ts (26 tests)
  * Registry-based and dynamic breaking change detection
  * Property additions/removals/requirement changes
  * Severity calculation and change merging
  * Nested property handling and recommendations

- post-update-validator.test.ts (24 tests)
  * Post-update guidance generation
  * Required actions and deprecated properties
  * Behavior change documentation (Execute Workflow, Webhook)
  * Migration steps, confidence calculation, time estimation

Also update README.md to include the new n8n_workflow_versions tool
in the Workflow Management tools section.

Coverage impact:
- Targets services with highest missing coverage from Codecov report
- Addresses 1630+ lines of missing coverage in new services
- Comprehensive mocking of dependencies (database, API clients)
- Follows existing test patterns from workflow-auto-fixer.test.ts

All tests use vitest with proper mocking, edge case coverage, and
deterministic assertions following project conventions.

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

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

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

README.md

@@ -981,6 +981,7 @@ These powerful tools allow you to manage n8n workflows directly from Claude. The
 - **`n8n_list_workflows`** - List workflows with filtering and pagination
 - **`n8n_validate_workflow`** - Validate workflows already in n8n by ID (NEW in v2.6.3)
 - **`n8n_autofix_workflow`** - Automatically fix common workflow errors (NEW in v2.13.0!)
+- **`n8n_workflow_versions`** - Manage workflow version history and rollback (NEW in v2.22.0!)
 
 #### Execution Management
 - **`n8n_trigger_webhook_workflow`** - Trigger workflows via webhook URL