Fix object schema missing properties error for tools without input parameters

[ezynda3]

Summary

Fixes #89: Tools created without input properties were causing OpenAI function calling validation errors.

Problem

When MCP tools were created without input parameters using mcp.NewTool("foo", mcp.WithDescription("tool without input")), the resulting OpenAPI schema had Type="object" but Properties=nil. This violated OpenAI's function calling schema requirements, causing the error:

OpenAIException - Invalid schema for function 'bar__foo' In context=(), object schema missing properties

Solution

• Added schema validation fix in loadServerTools to ensure object schemas have an empty properties map when Properties is nil
• The fix is minimal and targeted: if inputSchema.Type == "object" && inputSchema.Properties == nil { inputSchema.Properties = make(openapi3.Schemas) }

Testing

• Added comprehensive regression test TestIssue89_ObjectSchemaMissingProperties
• Added additional test coverage for tools without properties
• All existing tests continue to pass
• Project builds successfully with no linting issues

Impact

• ✅ Resolves OpenAI function calling validation errors for tools without input parameters
• ✅ Maintains backward compatibility
• ✅ Users no longer need the workaround of adding dummy __unused__ parameters
• ✅ No breaking changes to existing functionality

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