feat(models): add fee tiers to perps fee schedule

[kartojal]

Adds a PerpsFeeTier model (min_volume_30d, taker_fee_rate, maker_fee_rate) and a tiers tuple on PerpsFeeScheduleEntry, matching the updated GET /v1/info/fees contract. Exported from polymarket.models (root export omitted, following the PerpsRiskTier precedent for nested tier models).

The public-reads integration test now also asserts the fee schedule returns tiers.

Part of DEV-432.

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Note

Low Risk
Additive Pydantic schema and export changes for fee parsing; no auth, trading, or withdrawal logic touched.

Overview
Aligns perps fee schedule models with the updated GET /v1/info/fees response by modeling volume-based tiers alongside the existing category-level maker/taker rates.

Introduces PerpsFeeTier (min_volume_30d, taker_fee_rate, maker_fee_rate) and adds a required tiers tuple on PerpsFeeScheduleEntry. PerpsFeeTier is exported from the perps and top-level model packages (same pattern as nested tier types like PerpsRiskTier).

The public perps integration test now asserts the live fee schedule includes at least one tier.

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