fix: satisfy clippy::manual_is_multiple_of (Rust 1.96 stable)

Stable clippy 1.96 added manual_is_multiple_of; pre-existing modulo checks
in openab-core (format.rs, pre_seed.rs) and openab-gateway (wecom.rs) fail
`clippy --workspace -D warnings`. Mechanical fix; unblocks CI. Unrelated to
the OAuth change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: canyugs

crates/openab-core/src/format.rs

@@ -319,7 +319,7 @@ mod tests {
         for (i, chunk) in chunks.iter().enumerate() {
             let fence_count = chunk.lines().filter(|l| l.starts_with("```")).count();
             assert!(
-                fence_count % 2 == 0,
+                fence_count.is_multiple_of(2),
                 "chunk {i} has unbalanced fences ({fence_count}):\n{chunk}"
             );
         }

crates/openab-core/src/pre_seed.rs

@@ -222,7 +222,7 @@ fn extract_zip_budgeted(
 
     for i in 0..file_count {
         // Cooperative deadline check per file
-        if i % 100 == 0 && Instant::now() >= deadline {
+        if i.is_multiple_of(100) && Instant::now() >= deadline {
             anyhow::bail!("hooks.pre_seed: timed out during extraction at entry {i}");
         }
 
@@ -286,7 +286,7 @@ fn extract_tarball_with_limits(data: &[u8], dest: &Path, deadline: Instant) -> a
         }
 
         // Cooperative deadline check every 10 files
-        if file_count % 10 == 0 && Instant::now() >= deadline {
+        if file_count.is_multiple_of(10) && Instant::now() >= deadline {
             anyhow::bail!("hooks.pre_seed: timed out during tarball extraction at entry {file_count}");
         }
 

crates/openab-gateway/src/adapters/wecom.rs

@@ -137,7 +137,7 @@ fn decrypt_message(
         .decode(encrypted)
         .map_err(|e| anyhow::anyhow!("base64 decode failed: {e}"))?;
 
-    if cipher_bytes.is_empty() || cipher_bytes.len() % 16 != 0 {
+    if cipher_bytes.is_empty() || !cipher_bytes.len().is_multiple_of(16) {
         anyhow::bail!("ciphertext length {} not a multiple of 16", cipher_bytes.len());
     }