connectors: validation branches return success instead of MCP errors

[suleimansh]

Part of #145.

Both connectors return { error: '...' } for user-facing validation failures (github get-file directory case connector-github/src/index.ts:148; drive folder / share cases connector-google-drive/src/index.ts:151,155,213,214). These normalize to a success McpToolResult (connectors/src/mountConnectors.ts:123), so an agent cannot distinguish "failed" from "data" via the MCP isError flag.

Fix: return McpResponse.error(...) for these validation branches so the failure is machine-detectable, and document the convention in the kit README so copiers follow it.