Fix read_cell_output incorrectly reporting all outputs as too large (#318148)

Two
 related issues caused even tiny notebook cell outputs (e.g. 5 bytes)
to be replaced with "Output is too large to be used as context".

1. Inverted size check in `RunNotebookCellOutput`: `getCharLimit` converts
   tokens to chars (×4), but it was applied to the byteLength side and
   compared against a token count. Compare byteLength against
   getCharLimit(tokenBudget / sizeLimitRatio) instead — the threshold was
   16× too small.

2. Remove `ReadCellOutput` from `toolsCalledInParallel`. Tools in that set
   are invoked eagerly with a sentinel `{ tokenBudget: 1 }` sizing on the
   premise that they don't consume sizing info. `RunNotebookCellOutput`
   does — it gates output on `sizing.tokenBudget` — so the sentinel made
   every non-empty output trip the size check. Letting it use the normal
   lazy path gives it the endpoint's real prompt budget.