fix(effects): use switchMap for list-fetch effects to prevent stale data

[alubbock]

Description

Replace mergeMap with switchMap in the four list-fetching NgRx effects that handle user-driven queries (filter changes, sort, pagination).

Motivation

With mergeMap, rapidly changing filters or sort order leaves multiple concurrent HTTP requests in flight simultaneously. Responses can arrive out of order and overwrite newer store state with stale results. For example, a slow first request completing after a faster second one would silently replace the correct data with stale results.

switchMap cancels the previous in-flight request when a new action arrives, so only the response to the latest query is ever committed to the store. Angular's HttpClient supports cancellation, so the superseded request is also dropped on the network side.

Fixes:

• fetchDatasets$ in datasets.effects.ts
• fetchFacetCounts$ in datasets.effects.ts
• fetchMetadataKeys$ in datasets.effects.ts
• fetchProposalDatasets$ in proposals.effects.ts

The inner mergeMap calls that dispatch multiple actions from a single HTTP response are intentionally unchanged.

Changes:

• Four one-word operator changes across datasets.effects.ts and proposals.effects.ts
• Four marble-based regression tests added (one per effect) using the "-a-b" / "--a|" pattern: two rapid actions with a delayed response, asserting only the second response reaches the store

Tests included

• Included for each change/fix?
• Passing? (Merge will not be approved unless this is checked)

Summary by Sourcery

Use cancellable NgRx effects for list-fetching actions to avoid stale data when queries change rapidly.

Bug Fixes:

• Prevent outdated dataset, facet count, metadata key, and proposal dataset responses from overwriting newer store state by switching list-fetch effects to use cancellable operators.

Tests:

• Add marble tests verifying that a second rapid fetch action cancels the in-flight request and only the latest response updates the store for each affected effect.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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