Apply lint fixes for CGS

[stephanos]

What changed?

• Applied testifylint, staticcheck, and gofix auto-fixes.
• Exact commands that were run:

.bin/golangci-lint-v2.9.0 run --allow-parallel-runners --concurrency 4 --fix --enable-only testifylint --build-tags disable_grpc_modules,test_dep --timeout 20m --config=.github/.golangci.yml
.bin/golangci-lint-v2.9.0 run --allow-parallel-runners --concurrency 4 --fix --enable-only staticcheck --build-tags disable_grpc_modules,test_dep --timeout 20m --config=.github/.golangci.yml
make fmt-gofix
make goimports
make fmt
git diff --check

• No manual or AI changes were made.
• Some fixes caused lint errors; those were reverted again.
• Changes were all reviewed by me.