docs(dart): add comprehensive Dart SDK documentation

Add complete documentation for the AG-UI Dart SDK including:
- Main SDK overview with quick start guide
- Client package documentation (AgUiClient, configuration)
- Core types documentation (RunAgentInput, Messages, Context, Tools, State)
- Events system documentation (lifecycle, text message, tool call, state events)
- Encoder/decoder documentation (binary protocol, compression, streaming)

Documentation follows the same structure and style as TypeScript and Python SDKs
for consistency across all language implementations.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: mattsp1290

docs/sdk/dart/client/client.mdx

@@ -0,0 +1,352 @@
+---
+title: "AgUiClient"
+description: "Main client class for AG-UI server connectivity"
+---
+
+# AgUiClient
+
+The `AgUiClient` class is the primary interface for connecting to AG-UI compatible servers. It handles HTTP communication, SSE streaming, binary protocol encoding/decoding, and provides a type-safe API for agent interactions.
+
+## Constructor
+
+```dart
+AgUiClient({
+  required AgUiClientConfig config,
+  http.Client? httpClient,
+  Encoder? encoder,
+  Decoder? decoder,
+})
+```
+
+### Parameters
+
+- `config` (required): Configuration object with server details
+- `httpClient` (optional): Custom HTTP client implementation
+- `encoder` (optional): Custom encoder for request serialization
+- `decoder` (optional): Custom decoder for response parsing
+
+## Methods
+
+### runAgent
+
+Executes an agent and returns a stream of decoded events.
+
+```dart
+Stream<BaseEvent> runAgent(
+  String agentId,
+  RunAgentInput input, {
+  Map<String, String>? headers,
+})
+```
+
+#### Parameters
+
+- `agentId`: Unique identifier for the agent
+- `input`: Agent input containing messages, context, and configuration
+- `headers`: Optional additional headers for this request
+
+#### Returns
+
+A `Stream<BaseEvent>` that emits protocol events as they arrive.
+
+#### Example
+
+```dart
+final input = SimpleRunAgentInput(
+  messages: [
+    UserMessage(id: 'msg_1', content: 'Hello, agent!'),
+  ],
+  context: {'sessionId': '12345'},
+);
+
+await for (final event in client.runAgent('chat-agent', input)) {
+  switch (event) {
+    case RunStartedEvent():
+      print('Agent started');
+    case TextMessageDeltaEvent(delta: final text):
+      print('Agent says: $text');
+    case RunFinishedEvent():
+      print('Agent finished');
+  }
+}
+```
+
+### runAgentRaw
+
+Executes an agent and returns raw SSE messages without decoding.
+
+```dart
+Stream<SseMessage> runAgentRaw(
+  String agentId,
+  RunAgentInput input, {
+  Map<String, String>? headers,
+})
+```
+
+#### Use Cases
+
+- Custom event processing
+- Debugging and logging
+- Performance optimization when decoding isn't needed
+
+#### Example
+
+```dart
+await for (final message in client.runAgentRaw('agent', input)) {
+  print('Raw event: ${message.event}');
+  print('Raw data: ${message.data}');
+}
+```
+
+### cancelAgent
+
+Cancels an active agent execution.
+
+```dart
+Future<void> cancelAgent(String agentId)
+```
+
+#### Parameters
+
+- `agentId`: The agent ID to cancel
+
+#### Behavior
+
+- Immediately closes the SSE connection
+- Cleans up resources
+- Causes the event stream to complete
+
+#### Example
+
+```dart
+// Start long-running agent
+final stream = client.runAgent('analysis-agent', input);
+
+// Set up listener with timeout
+final subscription = stream.listen(
+  (event) => processEvent(event),
+  onError: (error) => handleError(error),
+);
+
+// Cancel after 10 seconds
+Timer(Duration(seconds: 10), () async {
+  await client.cancelAgent('analysis-agent');
+  await subscription.cancel();
+});
+```
+
+### dispose
+
+Cleans up all resources held by the client.
+
+```dart
+void dispose()
+```
+
+#### Important
+
+- Call this when the client is no longer needed
+- Cancels all active streams
+- Closes HTTP client connections
+- Releases memory resources
+
+## Properties
+
+### config
+
+```dart
+final AgUiClientConfig config;
+```
+
+The configuration used to initialize the client. Read-only.
+
+### activeStreams
+
+```dart
+Map<String, bool> get activeStreams;
+```
+
+Returns a map of currently active agent IDs and their status.
+
+## Error Handling
+
+The client throws specific exceptions for different error scenarios:
+
+### AgUiClientError
+
+General client-side errors.
+
+```dart
+try {
+  await for (final event in client.runAgent('agent', input)) {
+    // Process events
+  }
+} on AgUiClientError catch (e) {
+  print('Client error: ${e.message}');
+  print('Error code: ${e.code}');
+}
+```
+
+### NetworkError
+
+Network connectivity issues.
+
+```dart
+on NetworkError catch (e) {
+  print('Network error: ${e.message}');
+  // Implement retry logic
+}
+```
+
+### ValidationError
+
+Input validation failures.
+
+```dart
+on ValidationError catch (e) {
+  print('Validation failed: ${e.message}');
+  print('Failed fields: ${e.fields}');
+}
+```
+
+### ServerError
+
+Server-side errors (5xx status codes).
+
+```dart
+on ServerError catch (e) {
+  print('Server error: ${e.statusCode}');
+  print('Message: ${e.message}');
+}
+```
+
+## Advanced Usage
+
+### Custom HTTP Client
+
+Provide a custom HTTP client for advanced scenarios:
+
+```dart
+import 'package:http/http.dart' as http;
+import 'package:http/retry.dart';
+
+final retryClient = RetryClient(http.Client());
+
+final client = AgUiClient(
+  config: AgUiClientConfig(baseUrl: 'http://localhost:8000'),
+  httpClient: retryClient,
+);
+```
+
+### Custom Encoding/Decoding
+
+Implement custom encoders/decoders for specialized formats:
+
+```dart
+class CustomEncoder implements Encoder {
+  @override
+  List<int> encode(RunAgentInput input) {
+    // Custom encoding logic
+    return utf8.encode(jsonEncode(input.toJson()));
+  }
+}
+
+class CustomDecoder implements Decoder {
+  @override
+  BaseEvent decode(List<int> data) {
+    // Custom decoding logic
+    final json = jsonDecode(utf8.decode(data));
+    return BaseEvent.fromJson(json);
+  }
+}
+
+final client = AgUiClient(
+  config: config,
+  encoder: CustomEncoder(),
+  decoder: CustomDecoder(),
+);
+```
+
+### Stream Transformations
+
+Transform the event stream for specific use cases:
+
+```dart
+// Filter only message events
+final messageStream = client
+    .runAgent('agent', input)
+    .where((event) => event is TextMessageEvent);
+
+// Collect all text into a single string
+final completeText = await client
+    .runAgent('agent', input)
+    .whereType<TextMessageDeltaEvent>()
+    .map((event) => event.delta)
+    .join();
+```
+
+### Concurrent Agents
+
+Run multiple agents concurrently:
+
+```dart
+final agent1 = client.runAgent('agent1', input1);
+final agent2 = client.runAgent('agent2', input2);
+
+// Process both streams
+await Future.wait([
+  agent1.forEach((event) => processAgent1(event)),
+  agent2.forEach((event) => processAgent2(event)),
+]);
+```
+
+## Performance Considerations
+
+### Connection Pooling
+
+The client reuses HTTP connections when possible. For high-throughput scenarios:
+
+```dart
+final httpClient = http.Client();
+// Configure connection pooling
+final client = AgUiClient(
+  config: config,
+  httpClient: httpClient,
+);
+```
+
+### Memory Management
+
+For long-running streams:
+
+1. Process events immediately rather than buffering
+2. Cancel streams when no longer needed
+3. Dispose of the client when done
+
+### Binary Protocol
+
+The binary protocol is more efficient than JSON for large payloads:
+
+```dart
+// Binary protocol is used automatically when supported
+final stream = client.runAgent('agent', input);
+```
+
+## Testing
+
+Mock the client for unit tests:
+
+```dart
+class MockAgUiClient implements AgUiClient {
+  @override
+  Stream<BaseEvent> runAgent(String agentId, RunAgentInput input) {
+    return Stream.fromIterable([
+      RunStartedEvent(runId: 'test-run'),
+      TextMessageStartedEvent(messageId: 'msg-1'),
+      TextMessageDeltaEvent(messageId: 'msg-1', delta: 'Hello'),
+      TextMessageFinishedEvent(messageId: 'msg-1'),
+      RunFinishedEvent(runId: 'test-run'),
+    ]);
+  }
+}
+```