This change ensures that the OpenTelemetry context is correctly propagated across asynchronous boundaries throughout the ADK, primarily within RxJava streams.

### Key Changes

*   **Context Propagation:** Replaces manual `Scope` management (which often fails in reactive code) with `.compose(Tracing.withContext(context))`. This ensures the OTel context is preserved when work moves between different threads or schedulers.
*   **`Runner` Refactoring:**
    *   Adds a top-level `"invocation"` span to `runAsync` and `runLive` calls.
    *   Captures the context at entry points and propagates it through the internal execution flow (`runAsyncImpl`, `runLiveImpl`, `runAgentWithFreshSession`).
*   **`BaseLlmFlow` & `Functions`:** Updates preprocessing, postprocessing, and tool execution logic to maintain context. This ensures that spans created within tools or processors are correctly parented.
*   **`PluginManager`:** Ensures that plugin callbacks (like `afterRunCallback` and `onEventCallback`) execute within the captured context.
*   **Testing:** Adds several unit tests across `BaseLlmFlowTest`, `FunctionsTest`, `PluginManagerTest`, and `RunnerTest` that specifically verify context propagation using `ContextKey` and `Schedulers.computation()`.

### Files Modified
*   **`BaseLlmFlow.java`**, **`Functions.java`**, **`PluginManager.java`**, **`Runner.java`**: Core logic updates for context propagation.
*   **`LlmAgentTest.java`**, **`BaseLlmFlowTest.java`**, **`FunctionsTest.java`**, **`PluginManagerTest.java`**, **`RunnerTest.java`**: New tests for OTel integration.
*   **`BUILD` files**: Updated dependencies for OpenTelemetry APIs and SDK testing.

PiperOrigin-RevId: 884998997