Fix block headers URL when count is omitted

[ColumbusLabs]

Summary

• Fix getBlockHeaders() so omitting the optional count argument calls /block/headers/{height} instead of /block/headers/{height}/undefined.
• Preserve the counted form so getBlockHeaders(10, 2) still calls /block/headers/10/2.
• Add a mocked URL regression test covering both request shapes.

Evidence

• BitgesellOfficial/bglapiserver registers both /rest/block/headers/{block_pointer} and /rest/block/headers/{block_pointer}/{count}.
• The bglapiserver API docs mark count as not required for block headers.

Validation

• npm ci --legacy-peer-deps
• npm run build
• npm test -- --runTestsByPath src/__tests__/blockchain-url.test.ts --coverage=false
• git diff --check

Submitted for the Bitgesell bounty/improvement program: BitgesellOfficial/bitgesell#81. Payment details can be provided after review/approval.

[MyTH-zyxeon]

Maintainer review-assist for BitgesellOfficial/bitgesell#81:

This patch looks directionally correct and stays narrow: it removes the /undefined suffix when count is omitted, preserves the counted route, and adds a focused regression test for both request shapes.

Suggested acceptance checks:

1. Confirm the SDK route now matches the two visible bglapiserver shapes: /rest/block/headers/{block_pointer} and /rest/block/headers/{block_pointer}/{count}.
2. Verify the mocked regression test is enough for this bug class, since the failure mode is URL construction rather than response parsing.
3. Check whether any adjacent optional-path helpers in this SDK still interpolate undefined into REST paths, so the maintainer can batch similar route fixes if needed.
4. Preserve the scope as a route-construction fix plus regression coverage only; no broader API behavior change seems necessary from the visible diff.

If those checks pass, this looks like a clean candidate for the #81 review queue.

[ColumbusLabs]

Hi maintainers — this focused URL-construction fix is still current with main and includes regression coverage for both the counted and omitted-count routes. Could someone take a look when convenient? I am happy to make any requested adjustments.