[SDK] Add lex client seam and typed xrpc error matching (#11348)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/lib/lexClient.ts

@@ -0,0 +1,26 @@
+import {
+  type Agent,
+  type AgentOptions,
+  Client,
+  type ClientOptions,
+} from '@atproto/lex'
+
+/**
+ * App-standard factory for lex {@link Client}s. Use this instead of `new
+ * Client(...)` so every client shares the same lenient response processing.
+ *
+ * lex-client defaults to strict Lex processing, which rejects responses
+ * containing the LEGACY blob reference format (objects with `cid` and
+ * `mimeType` properties instead of `$type: 'blob'`). Older records on the
+ * network still carry these, and rejecting them would drop records the app
+ * currently renders. Lenient mode also relaxes datetime format checks (e.g.
+ * missing timezones) and blob MIME/size constraints. `Client.configure` only
+ * accepts `appLabelers` globally, so the option is defaulted here, per
+ * constructed client.
+ */
+export function createLexClient(
+  agent: Agent | AgentOptions,
+  options?: ClientOptions,
+): Client {
+  return new Client(agent, {strictResponseProcessing: false, ...options})
+}