[Repo Assist] test: add coverage for untested RandomAccessList and LazyList functions; fix zip empty-list bug

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Summary

Bug fix — RandomAccessList.zip throws on empty inputs

zip had an incorrect guard condition:

// Before
if randomAccessList1.Length = randomAccessList2.Length || randomAccessList1.IsEmpty then
    let arr = Array.create randomAccessList1.Length (randomAccessList1.[0], randomAccessList2.[0])

When both lists are empty, randomAccessList1.Length = 0 satisfies the first clause, but Array.create 0 (randomAccessList1.[0], ...) still eagerly evaluates randomAccessList1.[0], which throws. The fix handles the both-empty case explicitly before attempting array access:

// After
if randomAccessList1.IsEmpty && randomAccessList2.IsEmpty then empty
elif randomAccessList1.Length = randomAccessList2.Length then
    let arr = Array.create randomAccessList1.Length (randomAccessList1.[0], randomAccessList2.[0])
```

The old `|| randomAccessList1.IsEmpty` clause is also removed: if only one list is empty, the lengths differ and the `invalidArg` path is correct.

### New tests — `RandomAccessList`

Functions with no existing tests: `singleton`, `zip`, `reduce`, `map2`.

- `singleton` — basic length/head check
- `zip` — equal-length lists, both-empty (was the bug), different lengths (expect `ArgumentException`)  
- `reduce` — sum of elements, single element, empty list (expect `ArgumentException`)
- `map2` — element-wise combination, output length

### New tests — `LazyList`

Functions with no existing tests: `rev`, `concat`, `split`.

- `rev` — empty list, singleton, multi-element
- `concat` — empty outer list, non-empty inner lists, inner lists containing empties
- `split` — documents the intended contract: `split ll i` returns the first `i` elements **reversed** plus the elements **after** index `i`, making it a deletion helper used in `RealTimeDeque` and `BankersDeque` (`append (List.rev left) right` removes element `i`)

## Test Status

```
Passed!  - Failed: 0, Passed: 724, Skipped: 6, Total: 730

Previously 710 tests. 14 new tests added, all passing. Build has 0 errors.

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[dsyme]

/repo-assist add comprehensive testing for this, and if you spot any missing unit tests for other things add those too