fix(valkey): install haystack from main in test env for async mixin classes

Revert main dependency to haystack-ai>=2.26.1 (fixes license checker).
Override haystack in the test env with the main branch which contains
the async mixin classes (document_store_async) not yet in a stable release.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: SyedShahmeerAli12

integrations/valkey/pyproject.toml

@@ -26,7 +26,7 @@ classifiers = [
   "Programming Language :: Python :: Implementation :: CPython",
   "Programming Language :: Python :: Implementation :: PyPy",
 ]
-dependencies = ["haystack-ai>=2.27.0rc1", "valkey-glide>=2.2.0", "valkey-glide-sync>=2.2.0"]
+dependencies = ["haystack-ai>=2.26.1", "valkey-glide>=2.2.0", "valkey-glide-sync>=2.2.0"]
 
 [project.urls]
 Documentation = "https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/valkey#readme"
@@ -59,6 +59,7 @@ dependencies = [
     "pytest-asyncio",
     "pytest-cov",
     "pytest-rerunfailures",
+    "haystack-ai @ git+https://github.com/deepset-ai/haystack.git@main",
     "mypy",
     "pip"
 ]