Add fuzz tests to `pkg/scale`

## Task summary

#2629 was found randomly by adding a bad test case when writing tests for the trie proofs; this should not be found randomly and we should make use of fuzz tests instead. This is especially more important since we export it as `pkg/scale`, so other projects would be potentially affected.

- Add fuzz tests to check for panics for the decoder
- Add fuzz tests for the encoder + decoder
- Add fuzz tests to CI workflow

Start in pkg/scale by locating the decoder and encoder entry points and review existing trie-proof tests, especially the regression context from #2629. Add fuzz coverage for decoder panics and encoder/decoder behavior, then update the CI workflow so the fuzz tests run there; done means the tests execute successfully in CI.

Written by the indexing model from the issue text.