docs: add godoc comments to core exported types and functions

[aicontentcreate2023-star]

Fixes #18

Summary

Add comprehensive godoc comments to improve code documentation and discoverability. Comments follow Go conventions and provide clear descriptions of types, fields, and functions.

Changes

internal/storage/interface.go

• ✅ Client type and all 8 fields
• ✅ AuthorizationCode type and all 9 fields
• ✅ AccessToken type and all 5 fields
• ✅ RefreshToken type and all 5 fields
• ✅ Store interface and all 16 methods
• ✅ Config type and all 7 fields

internal/errors/errors.go

• ✅ AppError type and all 4 fields
• ✅ Error(), Unwrap(), New(), Wrap() functions
• ✅ All 7 error constructor functions
• ✅ All 7 predefined error variables

internal/config/config.go

• ✅ Config type and all 30+ fields (server, logging, CORS, OAuth, PostgreSQL)
• ✅ Load(), Validate(), GetServerAddress(), GetPostgresConnectionString(), HasSSL(), GetSessionSecret() functions

Coverage

• 3 files updated with complete godoc coverage
• All exported types documented
• All struct fields documented with purpose and constraints
• All interface methods documented with behavior and return values
• All functions documented with parameters and return values

Benefits

• ✅ Improved go doc output
• ✅ Better IDE autocomplete hints
• ✅ Easier onboarding for new contributors
• ✅ Clearer API contracts

Testing

# Verify documentation is accessible
go doc github.com/NP-compete/gomcp/internal/storage Client
go doc github.com/NP-compete/gomcp/internal/errors AppError
go doc github.com/NP-compete/gomcp/internal/config Config

Notes

This PR addresses the 3 highest-priority files mentioned in #18. Additional files (pkg/mcpprotocol/types.go, internal/mcp/server.go) can be added in a follow-up PR if needed.