compute/xla: table-driven dispatch tests for selectFMHAVariant and validation

Consolidate 8 separate TestSelectFMHAVariant_* functions into one
table-driven TestSelectFMHAVariant covering all cases: standard causal,
no-mask, f32/fp8 rejection, seqlen PADDING/PADDING_CAUSAL routing, bias
ScaleBias selection, bias+causal, and bias+seqlens NotImplemented.

Convert TestValidateSeqLen and TestValidateBias to the same pattern.
Add a Float16 valid-bias row to TestValidateBias to pin cast acceptance.
Coverage is identical; test count is unchanged.

Author: guygrigsby