test(bf-1pxdm): implement PDF fixture discovery for forms tests

Add discover_pdf_fixtures function to recursively find all PDF files in
tests/fixtures/forms/ directory using walkdir. Includes test that prints
discovered fixture names to stdout.

Acceptance criteria:
- walkdir is available in crates/pdftract-cli/Cargo.toml
- discover_pdf_fixtures function returns Vec<PathBuf> of discovered PDFs
- test_discover_pdf_fixtures runs and prints fixture names
- No compilation errors

Closes bf-1pxdm.

tests/forms_integration.rs

@@ -6,9 +6,52 @@
 //! - Widget annotation processing
 //! - Form data encoding/decoding
 
-// Placeholder module - tests will be added in subsequent beads
+use std::path::{Path, PathBuf};
+use walkdir::WalkDir;
 
-#[cfg(test)]
-mod tests {
-    // Test functions will be added here
+/// Discover all PDF files in the given directory recursively.
+///
+/// # Arguments
+/// * `fixtures_path` - Path to the fixtures directory to search
+///
+/// # Returns
+/// A `Vec<PathBuf>` containing paths to all discovered PDF files
+pub fn discover_pdf_fixtures<P: AsRef<Path>>(fixtures_path: P) -> Vec<PathBuf> {
+    let mut pdf_files = Vec::new();
+
+    let walker = WalkDir::new(fixtures_path)
+        .follow_links(false)
+        .into_iter()
+        .filter_map(|e| e.ok())
+        .filter(|e| {
+            e.file_type().is_file()
+                && e.path().extension().map(|ext| ext == "pdf").unwrap_or(false)
+        })
+        .map(|e| e.path().to_path_buf());
+
+    pdf_files.extend(walker);
+    pdf_files.sort();
+
+    pdf_files
+}
+
+#[test]
+fn test_discover_pdf_fixtures() {
+    let fixtures_dir = "tests/fixtures/forms";
+    let pdf_files = discover_pdf_fixtures(fixtures_dir);
+
+    println!("\n=== Discovered PDF Fixtures ===");
+    if pdf_files.is_empty() {
+        println!("No PDF files found in {}", fixtures_dir);
+    } else {
+        for pdf_path in &pdf_files {
+            println!("  - {}", pdf_path.display());
+        }
+        println!("Total: {} PDF file(s)", pdf_files.len());
+    }
+    println!("==============================\n");
+
+    // Test that the function runs without errors
+    // (We don't assert a count since fixtures may be added/removed)
+    let _ = pdf_files;
 }