Apply null-omitting JSON serialization to MCP stdio result responses and validate wire output

[Copilot]

Why make this change?

Strict MCP clients reject DAB stdio tool responses when optional content-block metadata is emitted as explicit null (for example, annotations: null and _meta: null). This causes otherwise successful tool calls to fail schema validation on strict clients.

What is this change?

• MCP stdio result serialization update

  • Updated McpStdioServer.WriteResult to serialize JSON-RPC result payloads with JsonIgnoreCondition.WhenWritingNull.
  • This omits null-valued properties in emitted result responses, including optional MCP content-block metadata fields when unset.
  • Existing request handling and content coercion behavior in CoerceToMcpContentBlocks remain unchanged.

• Wire-output regression coverage

  • Added McpStdioServerContentBlockTests coverage that captures actual stdio JSON-RPC output from WriteResult and verifies TextContentBlock output omits annotations and _meta when unset.
  • This ensures behavior is validated against emitted wire format, not just local serialization assumptions.

• Illustrative behavior

  // Before (strict clients reject):
  // { "type": "text", "text": "hello", "annotations": null, "_meta": null }
  
  // After:
  // { "type": "text", "text": "hello" }

How was this tested?

• Integration Tests
• Unit Tests

Sample Request(s)

• Example CLI usage:

  dotnet dab start --mcp-stdio role:anonymous --LogLevel Error --config dab.config.json

• Example MCP request/response shape:

  {"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"describe_entities","arguments":{}}}

  {
    "jsonrpc":"2.0",
    "id":1,
    "result":{
      "content":[
        { "type":"text", "text":"..." }
      ]
    }
  }

[benmccallum]

Nice, was about to suggest global options. Thanks for working on this so quickly. Looking forward to being able to get it working with Claude.

[Copilot]

Pull request overview

This PR aims to make MCP stdio JSON-RPC tool responses compatible with strict MCP clients by ensuring optional MCP content-block metadata is omitted instead of emitted as explicit null.

Changes:

• Added JSON serialization options to omit null-valued properties when writing JSON-RPC results to stdout.
• Added a unit test intended to validate that TextContentBlock output omits annotations and _meta when unset.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Azure.DataApiBuilder.Mcp/Core/McpStdioServer.cs	Applies WhenWritingNull serialization when emitting JSON-RPC results to stdout.
src/Service.Tests/UnitTests/McpStdioServerContentBlockTests.cs	Adds coverage around omitting annotations and _meta for a text content block.

[aaronburtle]

Looks good.

[RubenCerna2079]

LGTM! Thanks for addressing my concerns.