Description:
The askama indent filter does not apply the specified indentation (4 spaces) when the input string exceeds a certain length (e.g., 10,000 characters). Instead, the output string lacks the expected indentation.

Steps to Reproduce:

1. Create a project with the following Cargo.toml:

   [package]
   name = "indent-test"
   version = "0.1.0"
   edition = "2024"
   
   [dependencies]
   askama = { version = "0.14.0", features = ["code-in-doc"] }

2. Use the following lib.rs:

   use askama::Template;
   
   /// ~~~askama
   /// {{ text | indent(4) }}
   /// ~~~
   #[derive(Template)]
   #[template(ext = "txt", in_doc = true)]
   pub struct IndentTest {
       text: String,
   }
   
   pub fn run(limit: usize) {
       let rendered = IndentTest {
           text: format!("a\n{}", "b".repeat(limit - 2)),
       }
       .render()
       .unwrap();
   
       assert_eq!(&rendered[0..10], "a\n    bbbb");
   }
   
   #[test]
   fn test_9999() {
       run(9999);
   }
   
   #[test]
   fn test_10000() {
       run(10000);
   }

3. Run cargo test.

Expected Behavior:
The indent filter should prepend 4 spaces to each line after the first, regardless of input size. For test_10000, the first 10 characters of the rendered output should be "a\n bbbb".

Actual Behavior:
For inputs with 9,999 characters, the output is correct ("a\n bbbb"). For 10,000 characters, the indentation is missing, resulting in "a\nbbbbbbbb".

Environment:

• Rust edition: v1.86.0
• Askama version: 0.14.0