fix: generate TUI static files for both singular and plural variation names

[probelabs]

Problem / Task

The TUI static site generator in policy/tui.go explicitly strips the trailing s from the filename when generating static files. This means it only generates files under the v2/prod-variation/... path. However, tyk-pro uses VARIATION: prod-variations in its workflows, so it requests v2/prod-variations/... which returns a 404 Not Found, causing pipeline failures.

Changes

• Updated GenerateStatic in policy/tui.go to generate static files for both the singular and plural forms of the variation name (e.g., prod-variation and prod-variations).
• This maintains backward compatibility with both repositories.

Testing

• Verified that the code compiles successfully.
• Ran existing tests in gromit/policy and they pass.