Add env SegmentedReduce (non fixed-size overloads) (#7795)

* Add env SegmentedReduce

* Add env overloads for DeviceSegmentedReduce ArgMin/ArgMax and refactor to common impl

  - Add private segmented_reduce_impl that centralizes determinism
    validation (static_assert rejecting gpu_to_gpu), dispatch_with_env,
    and tuning extraction, eliminating boilerplate across all env overloads
  - Refactor Reduce, Sum, Min, Max env overloads to delegate to
    segmented_reduce_impl
  - Add new env overloads for ArgMin and ArgMax with full documentation
    including literalinclude snippet tags
  - Rewrite env_api tests covering all 6 APIs (Reduce, Sum, Min, Max,
    ArgMin, ArgMax) with determinism and stream_ref acceptance tests
  - Unify _env.cu and _env_launch.cu into a single _env.cu test file
    with default env, launch wrapper, custom stream, and tuning tests

* Add env overloads for fixed size segment APIs

* add env api literalinclude example just for Reduce and remove non guaranteed api test

* Remove fixed_size_segmented_reduce_impl underlying function as it added extra redundant logic for no reason

* Add unit tests for fixed-seg-size overloads and argmin argmax

* Fix GCC 7 auto deduction in generic lambda for fixed-size ArgMin/ArgMax env overloads

* Use __query_result_or_t to query tuning environment

* Static assert on numeric_limits specialization

* reviews

* Use explicit types for plus/minimum/maximum in env overloads to avoid integer promotion
* Add cuda::stream-based env to all env API tests

* Turn stream.wait() to stream.sync()

* Remove fixed-size overloads to simplify PR

* Fix breaking change from specializations

* Sum(env...) was already there, reintroduce it with the same constrains

* Address review nits for DeviceSegmentedReduce env overloads

  - Add missing non-overlap precondition to env ArgMin and ArgMax docs
  - Reorder env tests: group all env tests before custom stream tests
  - Add not_guaranteed determinism test for Reduce env API

* Add not_guaranteed query

* Docs nits

* Add run_to_run and not_guaranteed api tests

Author: gonidelis