when use new agent in workflow, if passing the variable from pervious node, it cannot exceed 2000 chars.

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### Dify version

16.1

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

use a long variable output from pervious node
refer the variable in the new agent, the variable output will be truncated.

### ✔️ Expected Behavior

It should not truncate user input at all.

### ❌ Actual Behavior

if over 2000 char. it will become [truncated]. And it is a hard code value in api/core/workflow/nodes/agent_v2/runtime_request_builder.py

```def _summarize_value(value: Any) -> str:
prompt_payload, used_download_mapping = WorkflowAgentRuntimeRequestBuilder._resolve_prompt_payload_value(value)
if used_download_mapping:
return json.dumps(prompt_payload, ensure_ascii=False, separators=(",", ":"))
text = str(value)
if len(text) > 2000:
return text[:2000] + "...[truncated]"
return text```

Start in api/core/workflow/nodes/agent_v2/runtime_request_builder.py at WorkflowAgentRuntimeRequestBuilder._summarize_value and reproduce the issue with a variable output longer than 2,000 characters. Done means a long value passed from a previous node is no longer truncated to 2,000 characters or replaced with [truncated].

Written by the indexing model from the issue text.