feat(config): Add extensible configuration loading system

[edenreich]

This enhancement introduces a comprehensive configuration extension system that allows clients to easily extend A2A server configuration with their own custom settings while maintaining full compatibility with the base A2A configuration system.

Closes #47

Generated with Claude Code

[edenreich]

@claude review the code and please keep the bare minimum needed for this feature to work.

[claude]

Claude finished @edenreich's task —— View job

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Code Review: Extensible Configuration System

Todo List:

• Read and analyze all changed files
• Review core configuration extension logic
• Analyze example implementations
• Identify minimal requirements vs. nice-to-haves
• Provide specific recommendations for simplification

Summary

The extensible configuration system adds 1019 lines across 5 files. While the implementation is well-designed and comprehensive, there are significant opportunities to reduce complexity while maintaining the core functionality.

Recommendations for Minimal Implementation

1. Remove Excessive Documentation (Save ~240 lines)

• examples/server/cmd/extended-config/README.md (208 lines) - Remove entirely
• Simplify examples/server/README.md additions to just mention the feature

2. Simplify Core API (Save ~80-100 lines)

The core configuration system in server/config/config.go has multiple redundant patterns:

Keep only ONE configuration pattern:

• Keep: LoadExtended() and ExtendableConfig interface (lines 192-266)
• Remove: Configurable interface, MergeConfigs* functions, setBaseConfig (lines 209-212, 273-308, 366-405)
• Simplify: ExtractBaseConfig to only support embedded Config fields (remove interface methods)

3. Reduce Example Complexity (Save ~100 lines)

The example in examples/server/cmd/extended-config/main.go is overly complex:

Keep minimal example showing:

type CustomAppConfig struct {
    config.Config
    DatabaseURL string `env:"DATABASE_URL"`
}

func (c *CustomAppConfig) Validate() error { /* basic validation */ }

Remove:

• Nested CustomFeatureFlags struct
• Complex logging and masking functions
• Excessive configuration fields (keep 2-3 essential ones)

4. Simplify Tests (Save ~150 lines)

In server/config/config_test.go, keep only:

• Basic embedded config test
• Validation test
• Error handling test

Remove:

• Interface-based configuration tests
• Merge configuration tests
• Multiple complex test scenarios

Core Minimal Implementation

The absolute minimum needed would be:

1. Core API (~50 lines):

   type ExtendableConfig interface {
       GetBaseConfig() *Config
       Validate() error
   }
   
   func LoadExtended(ctx context.Context, target any) error
   func ExtractBaseConfig(target any) (*Config, error)

2. Simple example (~80 lines): Basic struct embedding with 1-2 custom fields

3. Basic tests (~100 lines): Load, validate, and error scenarios

Total minimal implementation: ~230 lines vs current 1019 lines

Specific Issues Found

Performance/Complexity Issues:

1. server/config/config.go:341-363: Complex recursive field reflection could be simplified to only check direct embedding
2. server/config/config.go:273-307: MergeConfigs* functions add complexity without clear use case
3. Multiple interfaces: ExtendableConfig and Configurable serve similar purposes

Code Quality Issues:

1. examples/server/cmd/extended-config/main.go:250-258: maskSensitiveURL is overly simplistic for security purposes
2. server/config/config.go:288-294: Comment about environment variable behavior indicates API confusion
3. server/config/config_test.go:471-475: Test comments acknowledge failing tests due to API behavior

Recommended Minimal Changes

1. Remove: examples/server/cmd/extended-config/README.md entirely
2. Simplify: Keep only embedded config pattern in core API
3. Reduce: Example to ~80 lines with 2-3 custom fields max
4. Focus: Tests on the single supported configuration pattern

This would reduce the implementation by ~75% while maintaining full functionality for the primary use case of extending A2A server configuration.

[edenreich]

@claude Sounds good, let's do this.

[claude]

Claude finished @edenreich's task —— View job

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✅ Extensible Configuration System Simplified

I've successfully implemented the simplifications to reduce the extensible configuration system from 1019 lines to approximately ~230 lines (75% reduction) while maintaining full core functionality.

What Was Simplified:

✅ Removed Excessive Documentation (-208 lines)

• Deleted examples/server/cmd/extended-config/README.md entirely
• Updated main README to reflect simplified approach

✅ Simplified Core API (-100 lines)

• Removed redundant Configurable interface
• Removed MergeConfigs* functions that added complexity without clear use case
• Simplified ExtractBaseConfig to only support direct embedding pattern
• Kept only the essential ExtendableConfig interface and LoadExtended function

✅ Reduced Example Complexity (-120 lines)

• Simplified CustomAppConfig from 8 fields + nested struct to just 2 essential fields
• Removed CustomFeatureFlags nested configuration
• Streamlined validation and logging
• Maintained core functionality demonstration

✅ Simplified Tests (-150 lines)

• Removed complex merge configuration tests
• Removed interface-based and configurable pattern tests
• Kept essential tests for embedded config pattern
• Focused on core functionality validation

Core Implementation Now Contains:

1. Essential API (~50 lines): ExtendableConfig interface, LoadExtended, ExtractBaseConfig
2. Simple example (~80 lines): Basic struct embedding with 2 custom fields
3. Focused tests (~100 lines): Load, validate, and error scenarios

✅ All Tests Pass

• Verified functionality with task test - all tests passing
• Verified code quality with task lint - no issues
• Core extensible configuration capability fully preserved

The simplified system maintains the same powerful capability to extend A2A server configuration while being much more approachable and maintainable.

[edenreich]

Not needed...