fix: support lowercase proxy environment variables fallback

[fghpdf]

Description

This PR adds support for lowercase proxy environment variables as a fallback when uppercase versions are not set.

Changes

• Added fallback logic in SetDefault() method to check lowercase http_proxy, https_proxy, and no_proxy when uppercase versions (HTTP_PROXY, HTTPS_PROXY, NO_PROXY) are empty
• Added new helper function setDefaultStringFromEnv() to read from environment variables

Fixes

Fixes langgenius/dify#18752

Testing

• Code compiles successfully
• Follows the same pattern as other configuration defaults in the codebase

Behavior

• If HTTP_PROXY is set, it will be used (backward compatible)
• If HTTP_PROXY is empty but http_proxy is set, http_proxy will be used
• Same logic applies to HTTPS_PROXY/https_proxy and NO_PROXY/no_proxy

[gemini-code-assist]

Summary of Changes

Hello @fghpdf, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the application's proxy configuration handling by introducing a fallback mechanism for environment variables. It ensures that if standard uppercase proxy variables like HTTP_PROXY are not defined, the system will automatically check for and use their lowercase equivalents such as http_proxy. This change improves the robustness and flexibility of proxy settings, accommodating various system configurations without breaking existing behavior.

Highlights

• Proxy Environment Variable Fallback: Implemented a fallback mechanism to support lowercase proxy environment variables (http_proxy, https_proxy, no_proxy) if their uppercase counterparts are not explicitly set.
• New Helper Function: Introduced a new helper function, setDefaultStringFromEnv, to streamline the process of reading and applying environment variable values as defaults.
• Configuration Logic Update: Modified the SetDefault() method to incorporate the new fallback logic for proxy settings, ensuring broader compatibility for proxy configurations.

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[gemini-code-assist]

Code Review

This pull request correctly adds fallback support for lowercase proxy environment variables (http_proxy, https_proxy, no_proxy) when their uppercase counterparts are not set. The implementation is sound and achieves the goal described. I've included a couple of suggestions to refactor the changes by reusing an existing helper function. This would improve code maintainability and reduce duplication. Overall, this is a good and useful addition.

[gemini-code-assist]

To improve code reuse and maintainability, you can leverage the existing setDefaultString function here. This avoids introducing a new, specialized helper function (setDefaultStringFromEnv). The os.Getenv call is inexpensive, and this approach keeps the helper functions more generic.

I've added another comment suggesting the removal of the setDefaultStringFromEnv function definition, which would be redundant with this change.

Suggested change

	setDefaultStringFromEnv(&config.HttpProxy, "http_proxy")
	setDefaultStringFromEnv(&config.HttpsProxy, "https_proxy")
	setDefaultStringFromEnv(&config.NoProxy, "no_proxy")
	setDefaultString(&config.HttpProxy, os.Getenv("http_proxy"))
	setDefaultString(&config.HttpsProxy, os.Getenv("https_proxy"))
	setDefaultString(&config.NoProxy, os.Getenv("no_proxy"))

[gemini-code-assist]

Following the suggestion to use the existing setDefaultString function for setting proxy fallbacks, this new setDefaultStringFromEnv function is no longer needed and can be removed to reduce code duplication and improve maintainability.