Add tests for mdbook plugins

extensions/mdbook/test/plugins/BUILD.bazel

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+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_test")
+load("//:defs.bzl", "mdbook")
+
+rust_binary(
+    name = "mdbook-test-preprocessor",
+    srcs = ["mdbook_test_preprocessor.rs"],
+    edition = "2021",
+)
+
+mdbook(
+    name = "plugins",
+    srcs = glob(["src/**/*.md"]),
+    book = "book.toml",
+    plugins = [":mdbook-test-preprocessor"],
+)
+
+rust_test(
+    name = "plugins_test",
+    srcs = ["plugins_test.rs"],
+    data = [":plugins"],
+    edition = "2021",
+    rustc_env = {"MDBOOK_PLUGINS_OUTPUT": "$(rlocationpath :plugins)"},
+    deps = ["@rules_rust//rust/runfiles"],
+)
+
+build_test(
+    name = "plugins_build_test",
+    targets = [":plugins"],
+)

extensions/mdbook/test/plugins/book.toml

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+[book]
+authors = ["Plugin Test"]
+language = "en"
+src = "src"
+title = "Plugin Test Book"
+
+[preprocessor.test-preprocessor]
+command = "mdbook-test-preprocessor"

extensions/mdbook/test/plugins/mdbook_test_preprocessor.rs

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+use std::io::{self, Read};
+
+fn main() {
+    let args: Vec<String> = std::env::args().collect();
+    if args.len() > 1 && args[1] == "supports" {
+        std::process::exit(0);
+    }
+
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input).unwrap();
+
+    let modified = input.replace("{{secret}}", "42");
+
+    // mdBook preprocessors receive a JSON array containing `[context,
+    // book]` on stdin. They are expected to return only the `book`
+    // object on stdout.
+    //
+    // This is a very simple JSON parser that counts brackets to find
+    // the comma separating the two elements, mapping `[context,
+    // book]` to `book`.
+    let mut depth = 0;
+    let mut split_index = 0;
+    let bytes = modified.as_bytes();
+    for (i, &b) in bytes.iter().enumerate() {
+        if b == b'[' || b == b'{' {
+            depth += 1;
+        } else if b == b']' || b == b'}' {
+            depth -= 1;
+        } else if b == b',' && depth == 1 {
+            split_index = i;
+            break;
+        }
+    }
+
+    if split_index == 0 {
+        eprintln!("Failed to parse mdbook input.");
+        std::process::exit(1);
+    }
+
+    let book = &modified[split_index + 1..modified.len() - 1]; // Skip comma and last bracket
+    print!("{book}");
+}

extensions/mdbook/test/plugins/plugins_test.rs

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+use runfiles::{rlocation, Runfiles};
+use std::fs;
+
+#[test]
+fn test_preprocessor_output() {
+    let r = Runfiles::create().unwrap();
+
+    let dir = rlocation!(r, env!("MDBOOK_PLUGINS_OUTPUT")).unwrap();
+
+    let index_html = dir.join("index.html");
+    let content = fs::read_to_string(index_html).unwrap();
+
+    // Check if the preprocessor successfully replaced {{secret}} with 42
+    assert!(content.contains("The secret is: 42."));
+    assert!(!content.contains("{{secret}}"));
+}

extensions/mdbook/test/plugins/src/SUMMARY.md

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+# Summary
+
+[Index](index.md)

extensions/mdbook/test/plugins/src/index.md

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+# Plugin Test
+
+The secret is: {{secret}}.