fix(core): preserve git-memory formatting and enforce lock conflicts (#9451)

* test(core): strengthen git-memory system prompt stability integration coverage

Switch git-memory HTTP integration tests to OpenAI model handles and add assertions that system prompt content remains stable after normal turns and direct block value updates until explicit recompilation or reset.

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* fix(core): preserve git-memory formatting and enforce lock conflicts

Preserve existing markdown frontmatter formatting on block updates while still ensuring required metadata fields exist, and make post-push git sync propagate memory-repo lock conflicts as 409 responses. Also enable slash-containing core-memory block labels in route params and add regression coverage.

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* fix(memfs): fail closed on memory repo lock contention

Make memfs git commits fail closed when the per-agent Redis lock cannot be acquired, return 409 MEMORY_REPO_BUSY from the memfs files write API, and map that 409 back to core MemoryRepoBusyError so API callers receive consistent busy conflicts.

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* chore(core): minimize git-memory fix scope to memfs lock and frontmatter paths

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* chore: drop unrelated changes and keep memfs-focused scope

Revert branch-only changes that are not required for the memfs lock contention and frontmatter-preservation fix so the PR contains only issue-relevant files.

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* fix(memfs): lock push sync path and improve nested sync diagnostics

Serialize memfs push-to-GCS sync with the same per-agent Redis lock key used by API commits, and add targeted post-push nested-block diagnostics plus a focused nested-label sync regression test for _sync_after_push.

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---------

Co-authored-by: Letta <noreply@letta.com>

tests/test_log_context_middleware.py

@@ -1,7 +1,10 @@
+from unittest.mock import patch
+
 import pytest
 from fastapi import FastAPI
 from fastapi.testclient import TestClient
 
+import letta.server.rest_api.routers.v1.git_http as git_http_router
 from letta.log_context import get_log_context
 from letta.server.rest_api.middleware import LoggingMiddleware
 
@@ -35,6 +38,64 @@ def client(app):
 
 
 class TestLogContextMiddleware:
+    @pytest.mark.asyncio
+    async def test_sync_after_push_syncs_nested_block_labels_to_postgres(self, monkeypatch):
+        """Regression test: nested labels (e.g., system/human) are synced from git files."""
+
+        synced_calls = []
+
+        class DummyActor:
+            id = "user-123"
+            organization_id = "org-123"
+
+        class DummyGit:
+            async def get_files(self, agent_id, org_id, ref):
+                assert ref == "HEAD"
+                return {
+                    "system/human.md": "---\ndescription: human\nlimit: 20000\n---\nname: sarah",
+                    "system/persona.md": "---\ndescription: persona\nlimit: 20000\n---\nbe helpful",
+                }
+
+        class DummyMemoryRepoManager:
+            git = DummyGit()
+
+        class DummyBlockManager:
+            async def _sync_block_to_postgres(self, **kwargs):
+                synced_calls.append(kwargs)
+
+        class DummyAgentManager:
+            async def list_agent_blocks_async(self, **kwargs):
+                return []
+
+        class DummyUserManager:
+            async def get_actor_by_id_async(self, actor_id):
+                return DummyActor()
+
+        class DummyServer:
+            user_manager = DummyUserManager()
+            memory_repo_manager = DummyMemoryRepoManager()
+            block_manager = DummyBlockManager()
+            agent_manager = DummyAgentManager()
+
+        class DummyGitEnabledBlockManager(DummyBlockManager):
+            pass
+
+        dummy_server = DummyServer()
+        dummy_server.block_manager = DummyGitEnabledBlockManager()
+
+        monkeypatch.setattr(git_http_router, "_server_instance", dummy_server)
+
+        from letta.settings import settings as core_settings
+
+        monkeypatch.setattr(core_settings, "memfs_service_url", "http://memfs.test")
+
+        with patch("letta.services.block_manager_git.GitEnabledBlockManager", DummyGitEnabledBlockManager):
+            await git_http_router._sync_after_push(actor_id="user-123", agent_id="agent-123")
+
+        labels = {call["label"] for call in synced_calls}
+        assert "system/human" in labels
+        assert "system/persona" in labels
+
     def test_extracts_actor_id_from_headers(self, client):
         response = client.get("/v1/agents/agent-123e4567-e89b-42d3-8456-426614174000", headers={"user_id": "user-abc123"})
         assert response.status_code == 200