Describe the bug

Under FilterPolicy.MERGE, when the init and runtime filters constrain the same field, the result depends on how the filters are wrapped rather than on what they say.

• If either side is a bare comparison filter, the runtime condition replaces the init condition on that field, and a warning is logged. This matches the docstring of FilterPolicy.MERGE: "Runtime filters are merged with init filters, with runtime filters overwriting init values."
• If both sides are AND logical filters, combine_two_logical_filters concatenates the conditions. Both conditions on the field are kept, so the result is their intersection, and no warning is logged.

With the same intent (init meta.lang == "en", runtime meta.lang == "de"), three of the four shapes return the German document, and the fourth returns nothing without logging anything.

To Reproduce

from haystack import Document
from haystack.components.retrievers.in_memory import InMemoryBM25Retriever
from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack.document_stores.types import FilterPolicy

store = InMemoryDocumentStore()
store.write_documents([
    Document(content="hello world", meta={"lang": "en"}),
    Document(content="hello welt", meta={"lang": "de"}),
])

en = {"field": "meta.lang", "operator": "==", "value": "en"}
de = {"field": "meta.lang", "operator": "==", "value": "de"}
AND = lambda *c: {"operator": "AND", "conditions": list(c)}

for init, runtime in [(en, AND(de)), (AND(en), de), (en, de), (AND(en), AND(de))]:
    retriever = InMemoryBM25Retriever(store, filters=init, filter_policy=FilterPolicy.MERGE)
    docs = retriever.run(query="hello", filters=runtime)["documents"]
    print([d.meta["lang"] for d in docs])

Output on main (bc4729f):

['de']   # init comparison + runtime AND   (warning: init filter ignored)
['de']   # init AND + runtime comparison   (warning: runtime filter overwrites)
['de']   # init comparison + runtime comparison   (warning)
[]       # init AND + runtime AND   (no warning)

Expected behavior

I'm not sure which behavior you want here, so I'm asking before proposing a change. #12816 settled that two AND filters should both narrow the result. That reading conflicts with "runtime filters overwriting init values" and with the three comparison paths. Two consistent options:

1. In combine_two_logical_filters, when both are AND, drop init conditions whose field also appears in the runtime conditions, and log the same warning as combine_runtime_comparison_and_init_logical_filters.
2. Keep the intersection, and at least log a warning when the two AND filters share a field, so an empty result is not silent. The FilterPolicy.MERGE docstring would also need to be updated.

I'm happy to open a PR for whichever you prefer.

System:

• Haystack version: main @ bc4729f