feat(core): add response-side JSON-RPC error code classifier

[limehee]

Summary

This PR adds a public error-code classification utility to jsonrpc-core and aligns response-side validation and samples with that API.

Main changes:

• Added JsonRpcErrorClassifier for semantic classification of integer JSON-RPC error.code values
• Added JsonRpcErrorCodeCategory with:
  • STANDARD
  • SERVER_RESERVED_RANGE
  • CUSTOM
• Added DefaultJsonRpcErrorClassifier as the default implementation
• Refactored DefaultJsonRpcResponseValidator to reuse the classifier for:
  • STANDARD_ONLY
  • STANDARD_OR_SERVER_ERROR_RANGE
• Kept CUSTOM_RANGE as a direct numeric bounds check, independent of classifier category
• Added boundary and policy regression tests, including explicit STANDARD_ONLY rejection for custom codes
• Expanded the pure Java response-side sample to show parsed response validation plus error-code classification
• Updated README and protocol/pure-Java guides to document the new response-side utility

Related Issues

• Closes [Feature] Add JsonRpcErrorClassifier for standardized error code interpretation #24

Change Type

• Bug fix
• Feature
• Refactor
• Documentation
• Test
• Build/CI

JSON-RPC Impact

Describe protocol-level impact, if any:

• No protocol behavior change
• Request validation behavior changed
• Error mapping behavior changed
• Method registration/dispatch behavior changed

Details:

• This PR adds a reusable response-side utility for interpreting integer error.code values.
• Existing response validation policies remain the same in behavior.
• CUSTOM_RANGE still validates against configured numeric bounds rather than classifier category.
• CUSTOM classification does not mean invalid; it means the code is outside the standard set and the reserved server range.

Validation

• ./gradlew test
• ./gradlew check
• Added/updated tests for new behavior

Additional verification performed:

• ./gradlew :jsonrpc-core:test --tests 'com.limehee.jsonrpc.core.DefaultJsonRpcErrorClassifierTest' --tests 'com.limehee.jsonrpc.core.DefaultJsonRpcResponseValidatorTest' --no-daemon
• ./gradlew -p samples/pure-java-demo test --no-daemon

Documentation

• Updated README (if needed)
• Added migration notes for breaking changes (if any)

Notes:

• No breaking migration is required.
• The pure Java sample now demonstrates both response validation and semantic error-code classification.
• The protocol guide now explicitly documents standard codes, the reserved server range, and custom codes.