Remove unused oapi-codegen tool dependency

[snamber]

Summary

• remove the unused oapi-codegen tool dependency
• remove its version exclusions and tool-only transitive dependencies
• resolve Dependabot alert GHSA-rjwr-m7qx-3fjr by removing the vulnerable package from the module graph

The SDK was converted from generated to handcrafted code in 2024, when its go:generate entry point and generator configuration were deleted. The remaining tool declaration was therefore orphaned. Upgrading it to the patched v2.7.1 instead causes a YAML transitive-dependency compilation conflict, so retaining and upgrading an unused tool would require additional dependency constraints without restoring any repository functionality.

Verification

• go get ./...
• go mod tidy
• go mod verify
• go build ./...
• go test ./...
• go vet ./...
• confirmed github.com/oapi-codegen/oapi-codegen/v2 is absent from go list -m all
• git diff --check