fix: GetBlocks pagination off-by-one returning PerPage+1 results

[floflo777]

Summary

Fixes #197

GetBlocks was returning PerPage + 1 results instead of PerPage. The root cause is that page.PerPage += 1 inside the Load() callback mutates the same PerPage field that Load() uses to control iteration count, causing it to iterate one extra time beyond the requested page size. This also corrupts Count and TotalPages in the response.

Changes

• Create a separate PageParams with PerPage + 1 before calling Load(), so the iteration limit is set upfront without mutation inside the callback
• Use the original p.PerPage as the guard for results = append(results, block) so exactly PerPage blocks are returned
• Remove the page.PerPage += 1 mutation from inside the callback
• Recompute page.Count and page.TotalPages after Load() using the original PerPage value

Before

Request: PerPage=10
Response: Count=11, TotalPages=wrong

After

Request: PerPage=10
Response: Count=10, TotalPages=correct

The extra block is still fetched (needed to compute Took duration for the last visible block), but it is no longer included in the results or pagination metadata.

Test plan

• go build ./store/... passes
• go test ./store/... -v all 33 tests pass