refactor: make MockHttpClientCall internal

Summary

MockHttpClientCall is a public class purely because Ktor's HttpClientCall(client) base constructor requires it to be instantiable from the plugin's install code — it should be internal. This is independent of the OpenAPI migration (tracked in the epic, #72) and can be picked up any time. Good first issue: small, self-contained, mechanical.

Current state

MockHttpClientCall (devview-networkmock-ktor/src/commonMain/kotlin/com/worldline/devview/networkmock/ktor/plugin/NetworkMockPlugin.kt:356-408) is declared public class MockHttpClientCall(client: HttpClient) : HttpClientCall(client). It's part of the tracked public API surface (devview-networkmock-ktor/api/api.txt), but nothing about it is meant to be integrator-facing — it's an internal implementation detail of how the plugin fabricates a synthetic response. Its rawContent override is annotated @InternalAPI (:390-393), meaning it already depends on a Ktor API that isn't stable — being public compounds that risk, since any breaking change to Ktor's internals here would be a breaking change to this library's public API too, for a type integrators have no legitimate reason to construct themselves.

What to build

• Change MockHttpClientCall's visibility to internal.
• Verify this compiles — createMockHttpClientCall (:322-346), the only caller, is a private top-level function in the same file, so this should be a clean visibility narrowing with no call-site changes needed elsewhere.
• Confirm no test or sample code constructs MockHttpClientCall directly (check devview-networkmock-ktor/src/androidHostTest/... and sample/network/...) — if something does, that usage needs to move to a public entry point (or the test needs restructuring to not require direct construction).

Acceptance criteria

• MockHttpClientCall is internal, not public.
• devview-networkmock-ktor/api/api.txt is regenerated and no longer lists MockHttpClientCall.
• All existing tests still pass without needing to construct MockHttpClientCall directly.

Files likely touched

• devview-networkmock-ktor/src/commonMain/kotlin/com/worldline/devview/networkmock/ktor/plugin/NetworkMockPlugin.kt
• devview-networkmock-ktor/api/api.txt

Start in devview-networkmock-ktor/src/commonMain/kotlin/com/worldline/devview/networkmock/ktor/plugin/NetworkMockPlugin.kt, checking MockHttpClientCall and its createMockHttpClientCall caller. Check androidHostTest and sample/network for direct construction, then regenerate devview-networkmock-ktor/api/api.txt. Done means the class is internal, no direct external construction remains, and existing tests pass.