Remove incorrect assertion in fetchBatch

[Copilot]

High Level Overview of Change

Removes an incorrect assertion that assumed fetchBatch results size matches input hash count. The assertion fails for NullBackend which intentionally returns an empty vector.

Context of Change

The assertion was added in commit 4495952 based on a misunderstanding of move assignment semantics:

auto results = backend_->fetchBatch(batch).first;
XRPL_ASSERT(hashes.size() == results.size(), "...");  // WRONG

Move assignment replaces the entire vector, ignoring any initial size. The original code never pre-sized results, so the assertion assumption was invalid.

Production backends (MemoryFactory, RocksDBFactory, NuDBFactory) do return results.size() == hashes.size(), but NullBackend returns an empty vector by design. The assertion breaks for this valid implementation.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

None.

Before / After

Before:

auto results = backend_->fetchBatch(batch).first;
XRPL_ASSERT(hashes.size() == results.size(), "number of input hashes matches number of output objects");
for (size_t i = 0; i < results.size(); ++i) { ... }

After:

auto results = backend_->fetchBatch(batch).first;
for (size_t i = 0; i < results.size(); ++i) { ... }

Restores original behavior before the assertion was introduced.

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