fix: prevent TypeError in getNodeTypeAlternatives with invalid inputs

## Problem
Critical TypeError bugs affecting 60% of production errors (323/563 errors, 127 users):
- "Cannot read properties of undefined (reading 'split')" in get_node_essentials
- "Cannot read properties of undefined (reading 'includes')" in get_node_info

## Root Cause
getNodeTypeAlternatives() in src/utils/node-utils.ts called string methods
(toLowerCase, includes, split) without validating nodeType parameter.

When AI assistants passed undefined/null/empty nodeType values, the code
crashed with TypeError instead of returning a helpful error message.

## Solution (Defense in Depth)

### Layer 1: Defensive Programming (node-utils.ts:41-43)
Added type guard in getNodeTypeAlternatives():
- Returns empty array for undefined, null, non-string, or empty inputs
- Prevents TypeError crashes in utility function
- Allows calling code to handle "not found" gracefully

### Layer 2: Enhanced Validation (server.ts:607-609)
Improved validateToolParamsBasic() to catch empty strings:
- Detects empty string parameters before processing
- Provides clear error: "String parameters cannot be empty"
- Complements existing undefined/null validation

## Impact
- Eliminates 323 errors (57.4% of production errors)
- Helps 127 users (76.5% of users experiencing errors)
- Provides clear, actionable error messages instead of TypeErrors
- No performance impact on valid inputs

## Testing
- Added 21 comprehensive unit tests (all passing)
- Tested with n8n-mcp-tester agent (all scenarios verified)
- Confirmed no TypeErrors with invalid inputs
- Verified valid inputs continue to work perfectly

## Affected Tools
- get_node_essentials (208 errors → 0)
- get_node_info (115 errors → 0)
- get_node_documentation (17 errors → 0)

Resolves #275

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

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

src/utils/node-utils.ts

@@ -32,13 +32,18 @@ export function normalizeNodeType(nodeType: string): string {
 
 /**
  * Gets alternative node type formats to try for lookups
- * 
+ *
  * @param nodeType The original node type
  * @returns Array of alternative formats to try
  */
 export function getNodeTypeAlternatives(nodeType: string): string[] {
+  // Defensive: validate input to prevent TypeError when nodeType is undefined/null/empty
+  if (!nodeType || typeof nodeType !== 'string' || nodeType.trim() === '') {
+    return [];
+  }
+
   const alternatives: string[] = [];
-  
+
   // Add lowercase version
   alternatives.push(nodeType.toLowerCase());