refactor(params,common/countdown): consolidate exp-timeout validation rules #2877

Summary

The XDPoS v2 exponential-timeout rules are validated in two places that duplicate the same domain rules in two formulas and two numeric domains:

• params/config_backfill.go validateV2ExpTimeoutConfig (chain-config/genesis path): checks MaxExponent >= 32 against a literal 32, base^max_exponent >= 2^32, and — since fix(common/countdown,params): correct the exponential timeout arithmetic and validate its bounds #2836 — timeout_period * base^max_exponent against the time.Duration range using a float computation.
• common/countdown/exp_duration.go sanityCheck (runtime path, called from NewExpTimeoutDuration and SetParams): checks the same maxExponent and base^maxExponent rules against the maxExponentUpperbound constant, while GetTimeoutDuration saturates the product at math.MaxInt64 with integer arithmetic.

Future edits risk changing one side and missing the other. #2836 extended this pre-existing duplication by adding the duration-range rule on both sides in two idioms; it kept the duplication deliberately, per the minimal-change policy for that fix.

Proposal

Consolidate the rules in common/countdown — e.g. export a validation function covering the maxExponent upper bound, the base^maxExponent bound and the duration * base^maxExponent duration-range bound — and have validateV2ExpTimeoutConfig call it, keeping the params-side fieldPath error formatting.

Notes

• Pure maintenance, no runtime defect: both sides currently agree on every accepted configuration.
• Found while reviewing fix(common/countdown,params): correct the exponential timeout arithmetic and validate its bounds #2836.