[MCP] Centralize JSON‑RPC error handling, standardize MCP response envelope, and add default anonymous role for stdio mode.

[anushakolan]

Why make this change?

This is a follow up PR for, #2983 to address some of the newer comments.

What is this change?

1. Centralized JSON‑RPC error handling: all standard JSON‑RPC error codes are now defined in one place and used consistently throughout the MCP server (for things like parse errors, invalid requests, unknown methods, invalid parameters, and internal errors).
2. Standardized how responses are written: both successful results and errors now go through shared helpers, so every MCP response has a consistent JSON‑RPC envelope and is easy to change or audit.
3. Added an implicit default role to be anonymous when no role is provided in mcp-stdio mode.

How was this tested?

Tested manually by calling all the CRUD tools and execute tool.

Added an MCP server in VS Code in C:\DAB\data-api-builder\.vscode\mcp.json

"dab-with-exe": { "command": "C:\\DAB\\data-api-builder\\out\\publish\\Debug\\net8.0\\win-x64\\dab\\Microsoft.DataApiBuilder.exe", "args": ["start", "--mcp-stdio", "--config", "C:\\DAB\\data-api-builder\\dab-config.json"], "env": { "DAB_ENVIRONMENT": "Development" } }

1. When calling tools with Authenticated all tools pass.
2. When calling tools with Anonymous, all tools except describe-entities fail with permission denied error.
3. When calling tools without any role parameter, all tools except describe-entities fail with permission denied error.

[anushakolan]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 6 pipeline(s).

[Copilot]

Pull request overview

This PR is a follow-up to #2983 that improves the MCP stdio server implementation by centralizing JSON-RPC error handling, standardizing response writing, and adding an implicit default role when none is provided.

• Centralized all JSON-RPC error codes into a single JsonRpcErrorCodes class
• Standardized response handling through shared WriteResult and WriteError helper methods
• Added implicit default role of "anonymous" when --mcp-stdio is used without a role parameter

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Service/Utilities/McpStdioHelper.cs	Added [NotNullWhen(true)] attribute to enforce non-null role guarantee, implemented implicit default role of "anonymous", and removed unnecessary service scope creation since services are singletons
src/Service/Program.cs	Added copyright header (formatting artifact in diff)
src/Azure.DataApiBuilder.Mcp/Core/JsonRpcErrorCodes.cs	New file defining centralized JSON-RPC 2.0 error codes for consistent error handling across the MCP server
src/Azure.DataApiBuilder.Mcp/Core/McpStdioServer.cs	Replaced hardcoded error codes with constants from JsonRpcErrorCodes, refactored initialize handler to use standardized WriteResult, and updated server name in initialization response

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[Aniruddh25]

Approved, but please fix the title to mention "what" the code changes are. Follow up PR doesnt convey much information unless you look into the changes. The PR titles are seen directly in the Release notes, so they should be more meaningful. For example, in this case, it is refactor to use common JsonRpcErrorCodes