refactor(cache): redesign cache system with structured keys and improved performance

- Add repo_path parameter to cached_response and cached_vec_response functions
- Implement structured cache key format with namespace, repo_path, and request proto
- Replace global cache with Moka in-memory cache using weight-based eviction
- Set 256MB memory cap with 10-minute TTL and 2-minute TTI policy
- Add metrics collection for cache operations and evictions
- Implement efficient repo-scoped invalidation using key structure
- Add detailed documentation comments explaining cache architecture
- Remove outdated dependencies and update dependency versions
- Add error handling for encoding failures in cache operations
- Optimize Vec responses with length-delimited encoding and pre-allocation

repository/find_merge_base.rs

@@ -12,11 +12,21 @@ impl GitBare {
             return Ok(FindMergeBaseResponse::default());
         }
 
+        const MAX_MERGE_BASE_REVISIONS: usize = 64;
+        if request.revisions.len() > MAX_MERGE_BASE_REVISIONS {
+            return Err(crate::error::GitError::InvalidArgument(format!(
+                "too many revisions (max {MAX_MERGE_BASE_REVISIONS})"
+            )));
+        }
+
         let revisions: Vec<String> = request
             .revisions
             .iter()
             .map(|b| String::from_utf8_lossy(b).to_string())
             .collect();
+        for revision in &revisions {
+            crate::sanitize::validate_revision(revision)?;
+        }
 
         if revisions.len() < 2 {
             return Ok(FindMergeBaseResponse {