fix(sparse): declare SparseFloatArray dim from the max index, not the last one

[sainikhiljuluri]

What & why

sparse_rows_to_proto() builds the SparseFloatArray.dim incorrectly for a
dict/list sparse row whose indices are not in ascending order.

The row's indices are collected in input order (entity_helper.py:154-157),
and the byte payload is sorted before packing (sparse_float_row_to_bytes), so the
stored data is fine. But the dimension was taken from the last collected index:

if len(indices) > 0:
    row_dim = indices[-1] + 1     # last index, not the largest
dim = max(dim, row_dim)

For an unsorted row the last index is not the maximum, so dim is under-declared:

input row	max index	correct dim	old dim
{5: 0.1, 2: 0.3}	5	6	3
[(7, .1), (1, .2), (4, .3)]	7	8	5

Milvus then receives a dim that does not cover every index present in the row.

Fix

Use max(indices) + 1 (still guarded by the existing len(indices) > 0 check) so
the declared dim always covers the largest index regardless of input order. The
scipy code paths already derive dim from shape and are untouched.

Testing

Added test_sparse_rows_to_proto_dim_uses_max_index in
tests/unit/test_client_entity_helper.py with an unsorted dict row and an unsorted
list row. It fails on the previous code (assert 3 == 6) and passes with the fix.
Full tests/unit/test_client_entity_helper.py: 139 passed. ruff check clean.

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