fix: JSON-RPC ID round-trip preservation for numeric IDs

[Copilot]

This PR resolves an issue where JSON-RPC requests with numeric IDs were having their ID values converted to strings in responses, breaking the JSON-RPC specification requirement for ID preservation.

Problem

When sending a JSON-RPC request with a numeric ID (e.g., 123), the A2A server would return responses with the same value converted to a string (e.g., "123"). This violated the JSON-RPC 2.0 specification which requires that response IDs match the exact type and value of the request ID.

Before:

// Request
{ "jsonrpc": "2.0", "id": 123, "method": "tasks/get" }

// Response (incorrect - numeric ID became string)
{ "jsonrpc": "2.0", "id": "123", "result": "..." }

After:

// Request  
{ "jsonrpc": "2.0", "id": 123, "method": "tasks/get" }

// Response (correct - numeric ID preserved)
{ "jsonrpc": "2.0", "id": 123, "result": "..." }

Solution

Introduced a new JsonRpcId struct that preserves the original JSON type (string or number) of JSON-RPC request IDs:

• Type preservation: Uses an internal object? field to store either string or long values
• Type checking: Provides IsString, IsNumber, and HasValue properties
• Type-safe access: Offers AsString() and AsNumber() methods for retrieving typed values
• Custom serialization: Implements a JsonRpcIdConverter that maintains the original JSON type during serialization/deserialization
• Implicit conversions: Supports seamless conversion from string, int, and long types

Changes

• Created JsonRpcId struct with proper equality, conversion operators, and JSON converter
• Updated JsonRpcRequest and JsonRpcResponse to use JsonRpcId instead of string?
• Modified JsonRpcRequestConverter to preserve original ID types during deserialization
• Updated all response factory methods and streaming result classes to work with JsonRpcId
• Added backward compatibility extensions for A2AExceptionExtensions
• Updated source generation configuration to include the new type
• Fixed all existing tests and added comprehensive round-trip preservation tests

Testing

All 240 existing tests pass, plus new tests specifically verify:

• Numeric IDs (e.g., 123) remain numeric in responses
• String IDs (e.g., "test-id") remain strings in responses
• Null IDs are handled correctly
• Both request/response pairs and error scenarios preserve ID types
• Round-trip serialization/deserialization maintains original types

This change maintains full backward compatibility while ensuring compliance with the JSON-RPC 2.0 specification.

Fixes #113.

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[adamsitnik]

LGTM, but I wish we could just use some library for JSON-RPC related things..

[SergeyMenshykh]

I wonder if the conversion to string is necessary. Can't we just stick the requestId as is into the data dictionary without conversion? Then, new JsonRpcId(ex.GetRequestId()) won't be necessary.