The gate check forced operators to guess a per-call token size before
they could enable the TPM gate at all; est_tokens is now an optional
tuning override and effective_est_tokens() derives the reservation from
the provider quota. Reservation and settlement already read the same
derived value, so the deposit still nets to zero on both the success
path and the non-dead transient failure path.
The rest of _validate_gates is untouched, and the est_tokens field plus
its EST_TOKENS env key stay put for migration compatibility.
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