Bump Anthropic max_tokens default from 16384 to 65536

Prevents truncation of large tool calls (e.g. write_file with big
content) when neither agent config nor model catalog provides a value.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-llm/src/providers/anthropic.rs

@@ -1073,7 +1073,7 @@ fn build_api_request(
     let api_request = ApiRequest {
         model: request.model.clone(),
         messages: api_messages,
-        max_tokens: request.max_tokens.unwrap_or(16384),
+        max_tokens: request.max_tokens.unwrap_or(65536),
         system: system_value,
         temperature: request.temperature,
         top_p: request.top_p,