chore: import upstream snapshot with attribution

contributing/samples/core/app/README.md

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+# Application Configuration (App)
+
+## Overview
+
+This sample demonstrates how to configure an `App` in ADK. An `App` serves as the top-level container for an agentic system, wrapping a root agent or workflow and providing application-wide configurations such as plugins, event compaction, and context caching.
+
+## Sample Inputs
+
+- `Hello, who are you?`
+
+  *The application executes `greeter_agent`. The `CountInvocationPlugin` logs the agent and LLM run counts to the console.*
+
+- `Can you help me plan a trip?`
+
+  *As the conversation continues, event compaction automatically summarizes past turns every 2 invocations, while context caching optimizes token usage.*
+
+## Graph
+
+```mermaid
+graph TD
+    User[User Input] --> AppContainer[App: app]
+    subgraph AppContainer [App Container]
+        Plugins[Plugins: CountInvocation, SaveFilesAsArtifacts] --> GreeterAgent[root_agent: greeter_agent]
+        GreeterAgent --> Configs[Configs: EventCompaction, ContextCache]
+    end
+    AppContainer --> Response[User Response]
+```
+
+## How To
+
+### Wrapping an Agent in an App
+
+To configure application-level behaviors, instantiate an `App` object and pass your root agent to the `root_agent` parameter:
+
+```python
+from google.adk import Agent
+from google.adk.apps.app import App
+
+root_agent = Agent(
+    name="greeter_agent",
+    instruction="You are a friendly assistant.",
+)
+
+app = App(
+    name="app",
+    root_agent=root_agent,
+)
+```
+
+### Configuring Application Plugins
+
+Plugins provide cross-cutting capabilities (such as telemetry, custom logging, or artifact saving) across the entire application. Pass a list of plugin instances to `plugins`:
+
+```python
+from google.adk.plugins.save_files_as_artifacts_plugin import SaveFilesAsArtifactsPlugin
+
+app = App(
+    name="app",
+    root_agent=root_agent,
+    plugins=[
+        CountInvocationPlugin(),
+        SaveFilesAsArtifactsPlugin(),
+    ],
+)
+```
+
+### Configuring Event Compaction and Caching
+
+The `App` container is also where you define long-term session behavior and optimization strategies:
+
+- **`events_compaction_config`**: Manages token usage by periodically summarizing older turns in a session.
+- **`context_cache_config`**: Enables prompt caching across invocations to reduce latency and cost.
+
+```python
+from google.adk.apps.app import EventsCompactionConfig
+from google.adk.agents.context_cache_config import ContextCacheConfig
+
+app = App(
+    name="app",
+    root_agent=root_agent,
+    events_compaction_config=EventsCompactionConfig(
+        compaction_interval=2,
+        overlap_size=1,
+    ),
+    context_cache_config=ContextCacheConfig(
+        cache_intervals=10,
+        ttl_seconds=1800,
+        min_tokens=1000,
+    ),
+)
+```