Enable usestdlibvars linter

[aknuds1]

Enable usestdlibvars linter and fix issues.

[Copilot]

Pull Request Overview

This PR enables the usestdlibvars linter in the Go configuration and fixes all violations by replacing hardcoded HTTP method strings with the corresponding standard library constants.

• Enabled the usestdlibvars linter in the golangci-lint configuration
• Replaced hardcoded "GET" strings with http.MethodGet constant across test files
• Updated HTTP request creation calls to use standard library constants instead of string literals

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
.golangci.yml	Uncommented the usestdlibvars linter to enable it
server/static_file_server_test.go	Replaced hardcoded "GET" with http.MethodGet in test request creation
route/route_test.go	Updated multiple test functions to use http.MethodGet instead of "GET" string
config/http_config_test.go	Replaced "GET" strings with http.MethodGet constant in HTTP request tests

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