Anthropic message threads (#11545)

Anthropic can keep a conversation on the server (message threads beta).
A request can then continue from the previous reply and send only the
new messages, instead of the whole history every turn.

pi uses this when a session id is set and caching is on. It still builds
the full request, compares it with the previous one, and only sends the
new part if nothing earlier changed.

- Default: on only for the anthropic provider on api.anthropic.com.
  Gateways that drop unknown fields would lose the conversation.
  Override with compat.supportsMessageThreads.
- If the server rejects a continuation, pi resends the full request
  as a new thread. Other errors turn threads off for the session.
- New option StreamOptions.disableContinuation. The cache warmer sets
  it so warm requests don't replace the session's thread.
- cleanupSessionResources(sessionId) frees a session's thread state.

packages/agent/src/proxy.ts

@@ -52,6 +52,7 @@ type ProxySerializableStreamOptions = Pick<
 	| "reasoning"
 	| "cacheRetention"
 	| "sessionId"
+	| "disableContinuation"
 	| "headers"
 	| "metadata"
 	| "transport"
@@ -95,6 +96,7 @@ function buildProxyRequestOptions(options: ProxyStreamOptions): ProxySerializabl
 		reasoning: options.reasoning,
 		cacheRetention: options.cacheRetention,
 		sessionId: options.sessionId,
+		disableContinuation: options.disableContinuation,
 		headers: options.headers,
 		metadata: options.metadata,
 		transport: options.transport,