fix: move Otel testing library out production runtime

[abelonogov-ld]

Summary

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Note

Medium Risk
Medium risk because it changes the SDK’s public testing/inspection surface (replaces ObservabilityClient.getTelemetryInspector() with an ObservabilityOptions.telemetryInspector injection) and refactors core initialization to use ObservabilityService, which could affect downstream integrations if any relied on the old classes or runtime testing dependency.

Overview
Moves OpenTelemetry in-memory “telemetry inspection” support out of production runtime by making opentelemetry-sdk-testing a testImplementation/testFixtures dependency and introducing testing.InMemoryTelemetryInspector (test fixtures) for assertions.

Refactors telemetry inspection wiring: TelemetryInspector becomes an interface, a new ObservabilityOptions.telemetryInspector is added, and ObservabilityService composes the provided exporters into the debug composite exporters. The old ObservabilityClient/getTelemetryInspector() path is removed, and E2E tests are updated to inject and read the in-memory inspector via TestApplication.

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