fix: address review feedback on diagnostics multi-device discovery

[Copilot]

Addresses four code review issues from the diagnostics multi-device discovery PR: a silent failure path in desktop_peer, a use-after-free in diag_scanner, duplicated request logic in runtime.hpp, and missing test coverage for the explicit-target request overload.

Changes

• desktop_peer.cpp: Check enable_diagnostics() return value; log to stderr on failure instead of silently continuing without diagnostics.

• diag_scanner.cpp: Change devices from std::vector to std::deque. vector::push_back can reallocate, invalidating &dev pointers stored as callback context for in-flight async requests. deque::push_back guarantees pointer stability for existing elements.

• runtime.hpp: Extract private request_impl(const Addr& target, ...) helper; both public request overloads delegate to it, eliminating duplicated allocate/build/send logic.

// Both overloads now share one implementation path
std::optional<uint32_t> request(uint16_t service_id, ...) {
    Addr provider{};
    if (!sd_.get_provider_address(service_id, provider)) return std::nullopt;
    return request_impl(provider, service_id, ...);
}

std::optional<uint32_t> request(const Addr& target, ...) {
    return request_impl(target, service_id, ...);
}

• test_runtime.cpp: Add two tests for the explicit-target request overload — ClientRequest_ExplicitTarget_SendsWithoutSD (sends to target when SD has no provider) and ClientRequest_ExplicitTarget_ResponseCompletesCallback (response from target correctly fires callback).

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