fix: support lowercase proxy environment variables

[jcj46548-ux]

Summary

• read lowercase http_proxy, https_proxy, and no_proxy as fallbacks when uppercase proxy config is not set
• pass both uppercase and lowercase proxy variables to local plugin processes and dependency installation commands
• add tests covering lowercase fallbacks, uppercase precedence, and generated proxy environment entries

Tests

• go test ./internal/types/app
• go test ./internal/core/local_runtime -run '^$'

Full go test ./internal/core/local_runtime currently requires uv; the integration tests fail locally with UV not found.

Related issue: langgenius/dify#18752

[gemini-code-assist]

Code Review

This pull request refactors proxy environment variable handling by introducing helper methods (SetProxyEnvFallbacks and ProxyEnv) on the Config struct and updating the local runtime execution paths to use them. It also adds comprehensive unit tests for proxy configuration. The review feedback suggests adding nil checks to the new Config methods to prevent potential nil pointer dereferences, and clearing uppercase environment variables in the lowercase fallback test to avoid flakiness from the host environment.

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[gemini-code-assist]

The test TestConfigProxyLowercaseFallbacks can be flaky or fail if the host environment running the tests already has HTTP_PROXY, HTTPS_PROXY, or NO_PROXY set. Since envconfig.Process will populate cfg with those uppercase values first, the fallback logic won't be triggered, causing the assertions to fail. To make this test hermetic and independent of the host environment, explicitly clear the uppercase environment variables using t.Setenv before setting the lowercase ones.

Suggested change

	t.Setenv("http_proxy", "http://lowercase-http:8080")
	t.Setenv("https_proxy", "http://lowercase-https:8443")
	t.Setenv("no_proxy", "localhost,127.0.0.1")
	t.Setenv("HTTP_PROXY", "")
	t.Setenv("HTTPS_PROXY", "")
	t.Setenv("NO_PROXY", "")
	t.Setenv("http_proxy", "http://lowercase-http:8080")
	t.Setenv("https_proxy", "http://lowercase-https:8443")
	t.Setenv("no_proxy", "localhost,127.0.0.1")

[gemini-code-assist]

To prevent potential nil pointer dereference panics, it is safer to add a nil check for the config receiver before accessing its fields.

Suggested change

	func (config *Config) SetProxyEnvFallbacks() {
	if config.HttpProxy == "" {
	func (config *Config) SetProxyEnvFallbacks() {
	if config == nil {
	return
	}
	if config.HttpProxy == "" {

[gemini-code-assist]

To prevent potential nil pointer dereference panics, add a nil check for the config receiver before accessing its fields.

func (config *Config) ProxyEnv() []string {
	if config == nil {
		return nil
	}
	env := []string{}