refactor(scraper): Phase 2.4 — replace await-then-Match with functional operators

[jbarden]

Summary

Completes Phase 2 (Functional Paradigm Compliance) of docs/astar-dev-wallpaper-scraper/code-review-2026-07-10.v2.md by replacing the last await-then-Match patterns in the scraper workflows with functional operators.

• Adds OrElseAsync fallback operator to AStar.Dev.FunctionalParadigm (Task<Result<T,E>>.OrElseAsync(Func<TError, Task<Result<T,E>>> fallback)) — TDD: failing tests committed first, 4 new tests in GivenOrElseAsync.
• TopWallpapersWorkflow.LoadStartingPageAsync — OrElseAsync chain falling back to first page.
• SubscriptionsWorkflow.LoadStartingPageAsync — same OrElseAsync retry chain.
• SearchWorkflow.ProcessSearchCategoriesAsync — loop early-exit Match replaced with BindAsync accumulator; first failure short-circuits further category processing, behaviour unchanged.

Phase 2 items 5–7 were delivered in #726, #727 and #728; this PR delivers item 8, completing the phase.

Type of change

• Refactor
• Feature (new OrElseAsync operator in AStar.Dev.FunctionalParadigm)

Related issues / links

Closes #725. Relates to #729 (tracking issue for the new package operator).

How was this tested?

• Unit tests added/updated

GivenOrElseAsync: success skips fallback, failure invokes fallback with original error, fallback failure propagates.

Impacted areas

• packages/core/functional/AStar.Dev.FunctionalParadigm
• apps/desktop/Scraper/AStar.Dev.Wallpaper.Scraper (Workflows)

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TDD Checklist (required)

• Builds locally (dotnet clean && dotnet build --no-restore — 0 errors, 0 code warnings; only pre-existing NU1903 NuGet advisories in spikes/)
• Tests pass (dotnet test --no-build — FunctionalParadigm 135/135; scraper 419 pass / 16 fail and OneDrive 2135 pass / 21 fail are identical on main — zero new failures)
• No new analyzer warnings
• Public API changes reviewed (OrElseAsync added; no existing signatures changed)
• Documentation updated (if applicable)

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How to run tests locally

# Restore and run tests
dotnet restore AStar.Dev.slnx
dotnet test --verbosity normal

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Notes for reviewers

• SearchWorkflow accumulator: on failure BindAsync skips the binder, so remaining categories are not processed and the first error is returned — same semantics as the previous early return.
• Ensure CI passed for all OS runners where configured.

🤖 Generated with Claude Code

[github-actions]

Test results

980 tests  +4   963 ✅ +4   29s ⏱️ -11s
 11 suites ±0     1 💤 ±0 
 11 files   ±0    16 ❌ ±0 

For more details on these failures, see this check.

Results for commit 7bd91dd. ± Comparison against base commit bc25716.