fix: explicit return types and TS6 source compatibility fixes

[joker23]

This PR will tighten some type declarations to prepare this repo's upgrade to typescript 6.

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Note

Low Risk
Annotation-only and test-mock changes with no security or data-path logic altered.

Overview
Adds explicit return types to createOptions in the Fastly, Shopify Oxygen, Akamai Edgeworker, and server-edge SDKs (LDOptions or LDOptions & OxygenLDOptions), so merged options are typed consistently under stricter TypeScript inference.

For TS 6 / module interop, the EventProcessor test mock for EventSender now sets __esModule: true. In sortDataSet, the topological-sort loop uses a non-null assertion on unvisitedItems.values().next().value when the set is known to be non-empty.

No intended runtime behavior changes—typing and test compatibility only.

^{Reviewed by Cursor Bugbot for commit 1a0d9dc. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26365 bytes
Compressed size limit: 29000
Uncompressed size: 129044 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38739 bytes
Compressed size limit: 39000
Uncompressed size: 212244 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179654 bytes
Compressed size limit: 200000
Uncompressed size: 831422 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31979 bytes
Compressed size limit: 34000
Uncompressed size: 114243 bytes

[joker23]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 1a0d9dc. Configure here.}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.