feat(perps): expose server time

[kartojal]

DEV-467: Exposes the Perps server clock.

Tests: lint/format, typecheck, build, unit PASS; public live PASS.

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Note

Low Risk
Additive experimental read-only API with no auth or trading-path changes; main risk is callers depending on strict epoch-ms parsing behavior.

Overview
Adds get_server_time() on AsyncPublicClient and AsyncSecureClient, backed by a new Perps public action that calls GET /v1/info/time and returns an timezone-aware UTC datetime.

The JSON time field is parsed through a small response model that accepts epoch milliseconds only (reusing _require_epoch_ms); invalid shapes raise UnexpectedResponseError. datetime imports were moved out of TYPE_CHECKING so the public return type is runtime-resolvable.

Coverage includes mocked unit tests for parsing/endpoint path and a live integration test that the clock is within a few seconds of local UTC.

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