fix(pd): guard p_node before charging prefill load

[DivyaNarahari97]

_generate_one() dereferenced p_node to accumulate dispatched_prompt_chars one line above the "if not p_node or not d_node" check, so that guard could never run -- a None p_node raised AttributeError first, losing the intended error message and the abort path's context.

Moving the increment below the guard alone would introduce a second problem: the finally block decrements whenever p_node is not None and the load was not yet released, which after the move can be reached without the increment ever having happened (e.g. a valid p_node paired with a None d_node), silently skewing the node's in-flight load downward and biasing CacheAware dispatch toward it.

Replace the prefill_load_released flag with prefill_load_charged, which tracks the charge itself rather than an inference about it, so charge and release are symmetric by construction: incremented exactly where the charge happens, cleared exactly where it is refunded, and the finally block refunds if and only if a charge is still outstanding.