feat: add uploadpack.allowFilter support for GRASP-01 compliance

Add mandatory uploadpack.allowFilter capability to support partial clones
and fetches as required by GRASP-01 specification. This enables efficient
git operations for bandwidth-constrained clients (e.g., browser-based git
clients like git-natural-api).

Changes:
- Add uploadpack.allowFilter=true to git subprocess configuration
- Update SmartGitServer test helper with filter support
- Add integration tests for filter capability advertisement and functionality
- Update documentation to reflect filter as required capability

Tests verify:
- Filter capability is advertised in info/refs
- Filtered clones with blob:none work correctly
- Filtered fetches with tree:0 work correctly

1 files changed, 4 insertions, 0 deletions

diff --git a/tests/common/git_server.rs b/tests/common/git_server.rs
index 3190901..7634968 100644
--- a/tests/common/git_server.rs
+++ b/tests/common/git_server.rs
@@ -764,6 +764,8 @@ async fn handle_info_refs_upload_pack(
         .arg("uploadpack.allowReachableSHA1InWant=true")
         .arg("-c")
         .arg("uploadpack.allowTipSHA1InWant=true")
+        .arg("-c")
+        .arg("uploadpack.allowFilter=true")
         .arg("upload-pack")
         .arg("--advertise-refs")
         .arg("--stateless-rpc");
@@ -854,6 +856,8 @@ async fn handle_upload_pack(
         .arg("uploadpack.allowReachableSHA1InWant=true")
         .arg("-c")
         .arg("uploadpack.allowTipSHA1InWant=true")
+        .arg("-c")
+        .arg("uploadpack.allowFilter=true")
         .arg("upload-pack")
         .arg("--stateless-rpc");