fix(query): evict highest-cost node when MaxFrontierSize exceeded in shortest path

[veeceey]

Fixes #9577

The MaxFrontierSize feature introduced in #9333 has a bug in how it evicts nodes from the priority queue when the size limit is exceeded.

The current code calls pq.Pop() directly on the slice (not heap.Pop), which:

1. Removes the last element of the underlying array rather than the highest-cost item. In a min-heap, the last element's cost is arbitrary, so we might drop a low-cost node that's actually on the shortest path.
2. Doesn't maintain the heap invariant since it bypasses container/heap, corrupting the queue for subsequent heap.Push/heap.Pop calls.

The fix adds a removeMax() method that scans for the highest-cost item and removes it using heap.Remove(), which properly rebalances the heap. This ensures we always evict the least promising node while preserving the priority queue invariant and keeping shortest-path correctness.

Affects both the single shortest path (shortestPath) and k-shortest-path (runKShortestPaths) code paths.

All existing query package tests pass.

[veeceey]

Test Results

Query package tests all pass:

$ go test ./query/ -count=1
ok  github.com/dgraph-io/dgraph/v25/query  4.521s

Package builds cleanly:

$ go build ./query/...
# no errors

The systest/shortest-path tests require a running dgraph instance so I couldn't run those locally, but the fix is logically straightforward - replacing direct slice manipulation with proper heap removal of the max-cost node.