fix(server): return JSON-RPC errors for missing tools

[fahe1em1]

Summary

• keep protocol-level ProtocolErrors as JSON-RPC errors instead of wrapping them into CallToolResult.isError
• add a focused streamable HTTP test covering an unknown tool call
• preserve CallToolResult.isError wrapping for ordinary tool execution failures

Root Cause

McpServer already throws a ProtocolError when a tool does not exist, but the catch block in the tools/call handler was catching that protocol error and converting it into a normal tool result with isError: true.

Why This Is Small And Safe

This only changes how existing ProtocolErrors are surfaced. Tool execution errors still follow the existing CallToolResult.isError path, while protocol validation and missing-tool failures now stay aligned with JSON-RPC semantics.

Testing

• pnpm --filter @modelcontextprotocol/server test -- streamableHttp.test.ts
• pnpm --filter @modelcontextprotocol/server lint

Closes #1510.

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commit: b616fbf

[Bortlesboat]

This broadens the catch from only UrlElicitationRequired to all ProtocolError instances. Before this change, a ProtocolError with any other code (e.g. MethodNotFound thrown by user code inside a tool handler) would have been caught by the outer branch and returned as a CallToolResult with isError: true. Now it becomes a JSON-RPC error response instead.

Is this intentional for all protocol error codes? For example, if a tool implementation internally calls another protocol method that throws a ProtocolError, that would now surface as a JSON-RPC error rather than a tool-level error — which changes what the client sees. The test only covers the tool not found case (-32602), but this catch clause now affects every ProtocolError subtype.

Worth documenting which ProtocolError codes are expected to reach this path, or alternatively using an allowlist (e.g. UrlElicitationRequired, InvalidParams) rather than a blanket catch.