feat(gamma): add feeSchedule and feeType to Market

[jenish-25]

Closes #55.

Problem

The gamma API returns feeSchedule and feeType on every market, but gamma::types::response::Market does not model them. With the tracing feature enabled, serde_helpers logs a WARN per unknown field per market fetched:

WARN polymarket_client_sdk_v2::serde_helpers: unknown field in API response
  field=?.feeSchedule value={"exponent":1,"rate":0.07,"rebateRate":0.2,"takerOnly":true}
WARN polymarket_client_sdk_v2::serde_helpers: unknown field in API response
  field=?.feeType value="crypto_fees_v2"

For consumers that treat these WARNs as schema-drift signals, every gamma fetch produces noise that hides real drift.

Change

Adds a FeeSchedule struct and two fields to Market:

pub fee_schedule: Option<FeeSchedule>,
pub fee_type: Option<String>,

FeeSchedule follows the existing nested-type convention in this file (ClobReward, ImageOptimization): all-Option fields, #[non_exhaustive], and rename_all = "camelCase" — so no explicit #[serde(rename)] attributes are needed for feeSchedule/feeType/rebateRate/takerOnly.

Purely additive. Both fields are Option, so markets that omit them continue to deserialize unchanged, and Market is already #[non_exhaustive] so no downstream construction breaks.

Tests

Two tests added to tests/gamma.rs (httpmock, no network):

• market_fee_schedule_should_deserialize — asserts the field values from the issue's payload deserialize into typed values.
• market_without_fee_schedule_should_deserialize — asserts a market with neither field still deserializes, with both as None.

Verification

cargo test --all-features       554 passed, 0 failed
cargo clippy --all-targets --all-features -- -D warnings    clean
cargo fmt --check               clean

Note

The subfield types (exponent: i32, rate/rebateRate: Decimal, takerOnly: bool) are taken from the payload in #55. I could not reach gamma-api.polymarket.com from my network to sample a wider set of live markets, so if feeType is better modeled as an enum, or exponent is ever fractional, happy to adjust.

---

Note

Low Risk
Additive serde-only response modeling with optional fields; no API client or runtime behavior changes beyond deserialization.

Overview
Models Gamma API feeSchedule and feeType on Market so responses deserialize without unknown-field noise (notably with the tracing feature).

Adds a nested FeeSchedule type (exponent, rate, rebate_rate, taker_only) and optional fee_schedule / fee_type on Market, using the same camelCase / all-optional / #[non_exhaustive] pattern as other nested response types.

Tests: two httpmock market_by_slug cases—full fee payload typed as Decimal where expected, and a market with neither field leaving both None.

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