`index()` crashes with `RecursionError` on plain ibis backends

[espenjutte]

I encountered the following error when using BSL with plain ibis backend (in this case BigQuery). When trying to use .index it raises a RecursionError. Which is a shame since this would be very usefull when using with Bigquery as well.

Here is a Claude generated reproduction of this issue:

To reproduce

# model.yaml
orders:
  profile:
    type: bigquery
  table: orders
  database:
    - bigquery-public-data
    - thelook_ecommerce
  dimensions:
    order_status: _.status
  measures:
    order_count: _.count()

from boring_semantic_layer import from_yaml

models = from_yaml("model.yaml")
orders = models["orders"]
orders.index("order_status").execute()  # crashes

Traceback (abridged — full depth is ~3000 frames):

File ".../boring_semantic_layer/ops.py", line 4763, in execute
    return _rebind_to_canonical_backend(self.to_untagged()).execute()
File ".../boring_semantic_layer/ops.py", line 4744, in to_untagged
    fragments = [build_fragment(f) for f in fields_to_index]
File ".../boring_semantic_layer/ops.py", line 4725, in build_fragment
    _build_string_index_fragment(...)
File ".../boring_semantic_layer/ops.py", line 4532, in _build_string_index_fragment
    .aggregate(weight=weight_expr)
  [xorq deferred resolver repeated ~3000 more times]
  ...
RecursionError: maximum recursion depth exceeded

Root cause

_get_weight_expr in ops.py constructs the weight expression using
xo._.count() — xorq's vendored _ (Deferred). When this is passed as
weight_expr to .aggregate() on a plain ibis grouped table, ibis tries to
resolve the xorq Deferred, which creates another Deferred, which tries to
resolve... infinite recursion.

# ops.py:4511-4512 — weight_expr always built with xorq's _
def _get_weight_expr(base_tbl, by_measure, all_roots, is_string):
    if not by_measure:
        return xo._.count()   # <-- xorq Deferred, incompatible with plain ibis tables

The same problem appears in _build_string_index_fragment (lines 4537-4538)
and _build_numeric_index_fragment (lines 4555-4559) which also use
xo._["value"] — another xorq Deferred — when building the fragment selects
and filters.