Bound windowed GQA workspace allocations (#32602)
## Summary
- Size sliding-window GroupQueryAttention Flash split workspace using
the resident or staged KV extent instead of the unbounded absolute
sequence length.
- Size and launch the unfused fallback with the same effective KV
length.
- Preserve existing non-windowed behavior and all dispatch decisions.
- Add host coverage for the effective-length calculation and CUDA
regression coverage for `T >> C` on unfused and multi-token staged Flash
routes.
## Stack
Depends on #32454. The new change for this PR is commit `c9a579a6e8`;
earlier commits belong to the existing GQA workspace-recipe stack.
This is a runtime allocation-reduction prerequisite for the subsequent
GQA Level-1/Level-2 workspace-estimation PR. It does not add planner
opt-in, planned-pointer access, root slicing, or estimator wiring.
## Validation
- CUDA provider test target built successfully.
- CUDA internal module: 239 passed, 2 unrelated LeanAttention tests
skipped.
- Targeted GQA runtime tests: 5 passed, including windowed unfused
`T=64, C=8` and staged Flash `T=64, C=8, S=3`.
- Changed-file lintrunner passed.
- `git diff --check` passed.
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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
