feat: add config validation mode

[ptichalouf]

Summary

Adds a -check-config CLI flag that loads and validates the configured local or remote config, then exits without binding the HTTP server.

This is useful for CI, init containers, deployment scripts, and dashboards that want to validate generated mcp-proxy config before restarting a live proxy.

Changes

• Add -check-config flag.
• Print a concise success message with configured MCP server count.
• Document the flag in docs/USAGE.md.

Verification

• go test ./...
• go run . -config config.json -check-config

[gemini-code-assist]

Code Review

This pull request introduces a -check-config flag to load and validate the configuration file without starting the HTTP server, along with updated documentation in docs/USAGE.md. Feedback on the changes points out that the current implementation only loads the JSON file but fails to validate individual MCP server configurations. It is recommended to iterate through the configured servers and validate each one using parseMCPClientConfigV2 to ensure invalid configurations are caught.

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[gemini-code-assist]

The -check-config flag is intended to validate the configuration, but currently it only loads the JSON file and checks if mcpProxy is present. It does not validate the individual MCP server configurations (e.g., checking if a stdio transport has a command specified, or if the transport type is valid), which are validated via parseMCPClientConfigV2 during normal startup.

To ensure that invalid server configurations are caught during the config check, we should iterate over the configured servers and validate each one using parseMCPClientConfigV2.

	if *checkConfig {
		for name, server := range config.McpServers {
			if _, err := parseMCPClientConfigV2(server); err != nil {
				log.Fatalf("Failed to validate config for server %q: %v", name, err)
			}
		}
		fmt.Printf("Config OK: %d MCP server(s) configured\n", len(config.McpServers))
		return
	}

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Run the same per-server validation in check mode

Because this branch returns immediately after load(), -check-config skips the per-server validation that normal startup performs through startHTTPServer/newMCPClient/newMCPServer. For example, a config containing a server with transportType: "stdio" but no command will still print Config OK here, while normal startup fails in parseMCPClientConfigV2 with command is required for stdio transport, so CI or init containers can approve configs that the proxy cannot start with.

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