Fix multiline if let chains in gem_hypercore client

[Copilot]

Four if let guard chains in crates/gem_hypercore/src/rpc/client.rs were unnecessarily split across three lines, reducing readability.

Consolidated all occurrences to single-line form:

// Before
if let Some(dex) = dex
    && !dex.is_empty()
{
    payload["dex"] = json!(dex);
}

// After
if let Some(dex) = dex && !dex.is_empty() {
    payload["dex"] = json!(dex);
}

Affected methods: get_clearinghouse_state_with_dex, get_metadata_with_dex, get_open_orders_with_dex, get_perpetual_portfolio_with_dex.

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