Release prep: bump version to 2.5.2

[ChrisRackauckas]

Summary

Bumps HighDimPDE from 2.5.1 to 2.5.2. The last registered release (v2.5.1) corresponds to commit 9cd0e71 ("Bump version to 2.5.1 (#158)"), but the tree content has since changed without a version bump (registry tree-sha for 2.5.1 no longer matches HEAD's root tree).

Changes since v2.5.1 (commit 9cd0e71)

• 3d35e5b QA: run_qa v1.6 form + ExplicitImports
• 74cea11 build(deps): bump actions/checkout from 6 to 7
• fe18924 Use SciMLTesting 2.1 QA docs check (Use SciMLTesting 2.1 QA docs check #159)

Concretely, the diff (git diff 9cd0e71 HEAD) touches:

• src/HighDimPDE.jl, src/MCSample.jl: added docstrings for the existing PIDESolution struct and NoSampling struct (no behavior/API change).
• test/qa/qa.jl, test/qa/Project.toml, Project.toml: bumped the test-only SciMLTesting compat lower bound from 1.6 to 2.1 and adopted SciMLTesting's api_docs_kwargs QA option (test/CI tooling only, not a runtime dependency).
• test/GPU/gpu.jl, test/test_groups.toml: added a new GPU test group for CI (test infrastructure only).

No new exported API, no functional/runtime source changes, no breaking changes — everything is docs, tests, and CI.

Bump type: PATCH (2.5.1 -> 2.5.2)

Docs-only additions (docstrings) and test/CI-only changes qualify as a patch release per SemVer.

Compat bounds

SciMLTesting's compat lower bound was already raised from 1.6 to 2.1 in the diff itself (commit fe18924), consistent with its new api_docs_kwargs usage in test/qa/qa.jl. SciMLTesting is a test-only dependency ([extras]/[targets]), not a runtime dependency, so this does not affect downstream consumers. No other SciML-ecosystem compat bounds were touched, and none needed adjustment — no new APIs from SciMLBase, DiffEqBase, SciMLSensitivity, StochasticDiffEq, etc. are used in this diff.

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