google-cloud-bigtable: ValueBitmaskFilter support for data client

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### Summary of the issue

**Context**
We're on google-cloud-bigtable 2.38.0 using `BigtableDataClient` (the data client). We wanted to use the new `value_bitmask_filter` row filter (added to the `RowFilter` proto in 2.38) as part of a `ConditionalRowFilter` chain for a server-side bloom-filter prefilter.

**Expected Behavior:**
`google.cloud.bigtable.data.row_filters` exposes a `ValueBitmaskFilter` class that subclasses the data-client `RowFilter` ABC, so it composes into `RowFilterChain` / `ConditionalRowFilter` alongside the other data-client filters — matching the `ValueBitmaskFilter` that already exists in the legacy `google.cloud.bigtable.row_filters` module.

**Actual Behavior:**
`google.cloud.bigtable.data.row_filters` has no `ValueBitmaskFilter`. The legacy module's wrapper can't be used because it subclasses a different `RowFilter` base (`to_pb()` vs the data client's `_to_dict()`/`_to_pb()`), so it doesn't compose into data-client filter chains. We worked around it with a local `RowFilter` subclass overriding `_to_dict()` to return `{"value_bitmask_filter": data_v2_pb2.ValueBitmask(mask=...)}`, which works fine — but it'd be nice to have parity in the data-client module.

### API client name and version

google-cloud-bigtable 2.38

### Reproduction steps: code

n/a

### Reproduction steps: supporting files

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### Reproduction steps: actual results

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### Reproduction steps: expected results

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### OS & version + platform

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### Python environment

python 3.14

### Python dependencies

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### Additional context

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