[codex] Make mod-check use Go module proxy

[hieblmi]

Summary

• Update make mod-check to use GOPROXY=https://proxy.golang.org,direct instead of forcing GOPROXY=direct.
• Apply the same proxy setting to the root module, swapserverrpc, looprpc, and tools tidy checks.

Rationale

The direct-only module fetch path can hit transient Git shallow-clone cache failures for public transitive dependencies, such as modernc.org/ccgo/v3 via github.com/golang-migrate/migrate/v4. Using the public Go module proxy first avoids flaky direct VCS fetches while preserving direct as a fallback.

Validation

• make mod-check

[gemini-code-assist]

Summary of Changes

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This pull request improves the reliability of the module dependency check process by switching from a direct-only VCS fetch to using the public Go module proxy. This change helps mitigate transient network or cache issues encountered when fetching public transitive dependencies, while still maintaining direct access as a fallback.

Highlights

• Go Module Proxy Configuration: Updated the GOPROXY environment variable in the mod-check Makefile target to use https://proxy.golang.org,direct instead of direct.
• Improved Dependency Resolution Reliability: Enabled the public Go module proxy to mitigate transient Git shallow-clone failures when fetching public transitive dependencies.

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[gemini-code-assist]

Code Review

This pull request updates the GOPROXY configuration in the Makefile to use the official Go proxy before falling back to direct connections. A review comment suggests defining a variable for these settings to avoid repetition and improve maintainability.

[gemini-code-assist]

The GOPROXY configuration is repeated four times within the mod-check target. While this is explicit, it introduces a maintainability issue as any future changes to the proxy settings (e.g., adding a different mirror or changing the fallback behavior) would need to be updated in multiple places. Consider defining a Makefile variable for these settings to adhere to the DRY (Don't Repeat Yourself) principle.