[Flight] Fix double-incremented pending chunks counter (#31833)

Before calling `emitTimingChunk` inside of `forwardDebugInfo`, we must
not increment `request.pendingChunks`, as this is already done inside of
the `emitTimingChunk` function.

I don't have a unit test for this, but manually verified that this fixes
the hanging responses in https://github.com/vercel/next.js/pull/73804.