SEP-2164 compliance: return ResourceNotFound/InvalidParams error codes for unknown resources and prompts

[carldebilly]

Problem (2026-07-28 spec compliance, see modelcontextprotocol/csharp-sdk#1539)

SEP-2164 standardizes the resource not found error code, and the SDK 2.0 line exposes McpErrorCode.ResourceNotFound with McpProtocolException(message, code). Repl.Mcp currently throws code-less McpException for unknown lookups, which surfaces as generic InternalError (-32603):

• McpServerHandler.ReadResourceAsync: throw new McpException($"Unknown resource: {uri}") → should be McpErrorCode.ResourceNotFound (-32002).
• McpServerHandler.GetPromptAsync: throw new McpException($"Unknown prompt: {promptName}") → should be McpErrorCode.InvalidParams (-32602), the JSON-RPC code for prompts/get with a bad name.

Hosts branch on these codes to distinguish 'gone' from 'broken'.

Ready-made regressions

Two RED tests already exist (parked during PR #71 review to keep that PR scoped): When_ReadingUnknownResource_Then_ResourceNotFoundCodeIsReturned and When_GettingUnknownPrompt_Then_InvalidParamsCodeIsReturned — observed failures: InternalError -32603 in both cases.

Dependency

Requires the SDK 2.0 line (PR #71): McpProtocolException(message, McpErrorCode) does not exist in 1.4.1. Land as a small follow-up PR right after #71 merges.

Related compliance notes

• SEP-2106 (JSON Schema 2020-12): Repl's generated schemas only emit 2020-12-valid keywords — already conformant.
• SEP-2260 (server requests bound to a client request): satisfied by PR feat(mcp): migrate to ModelContextProtocol 2.0 line (2.0.0-preview.3) #71's per-request AsyncLocal binding.
• SEP-2567/SEP-2243 (sessionless state handles, HTTP standardization): stdio N/A; to revisit with the Streamable HTTP work (PR Add MCP Streamable HTTP hosting #27 / Define first-class MCP setup requirements for Repl.Mcp apps #44).