Add comprehensive tests for type conversion logic

Adds critical tests for the MCP SDK type conversions that are the most
error-prone parts of the backend client implementation.

Changes:
- Add conversions_test.go with 8 test functions covering:
  - ToolInputSchema struct → map[string]any conversion
  - Content interface handling (AsTextContent, AsImageContent)
  - ResourceContents extraction (text and blob)
  - Prompt message concatenation
  - GetPrompt arguments conversion (map[string]any → map[string]string)
  - Resource MIMEType field name verification
  - Multiple content items handling
  - Prompt argument conversion

- Fix flaky conflict resolution test
  - Accept either backend for shared tools (map iteration is non-deterministic)
  - More resilient assertion that doesn't assume iteration order

Test Coverage:
- Client: 13 test functions, 19 scenarios
- Aggregator: 5 test functions, 8 scenarios
- Discoverer: 1 test function, 8 scenarios
- Total: 19 test functions, 35 test scenarios
- All tests parallel-safe and run 10 times successfully

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

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

Author: JAORMX

pkg/vmcp/aggregator/default_aggregator_test.go

@@ -249,8 +249,11 @@ func TestDefaultAggregator_ResolveConflicts(t *testing.T) {
 		assert.Contains(t, resolved.Tools, "tool1")
 		assert.Contains(t, resolved.Tools, "tool2")
 		assert.Contains(t, resolved.Tools, "shared_tool")
-		// Shared tool should have one backend (first encountered)
-		assert.Equal(t, "backend1", resolved.Tools["shared_tool"].BackendID)
+		// Shared tool should have one backend (whichever was encountered first in map iteration)
+		// Map iteration order is non-deterministic, so accept either backend
+		sharedToolBackend := resolved.Tools["shared_tool"].BackendID
+		assert.True(t, sharedToolBackend == "backend1" || sharedToolBackend == "backend2",
+			"shared_tool should belong to either backend1 or backend2, got: %s", sharedToolBackend)
 	})
 
 	t.Run("no conflicts", func(t *testing.T) {