refactor(sdk-client): prefer interfaces over classes in datasource

[joker23]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

SDK-1706

Describe the solution you've provided

This PR refactors the datasource code to align with the CONTRIBUTING.md guidelines that prefer interfaces over classes for publicly exposed types.

Changes:

• Convert DataSourceState enum to a union type with a const object for backward compatibility
• Convert DataSourceEventHandler class to interface + factory function (createDataSourceEventHandler)
• Convert DataSourceStatusManager class to interface + factory function (createDataSourceStatusManager)
• Convert Requestor class to interface + factory function (createRequestor)
• Update imports to use named imports instead of default imports

The factory functions use closure-based state management to maintain private state without class fields. Default exports are preserved for backward compatibility.

Describe alternatives you've considered

Could have kept classes and only exposed interfaces, but the factory function pattern is cleaner and aligns better with the guidelines about bundle size optimization (functions minify better than class methods).

Additional context

All 272 unit tests pass, and browser SDK tests (96 tests) also pass. Lint checks pass.

Human review checklist

• Verify DataSourceState const object values match the original enum values
• Confirm the eslint-disable for @typescript-eslint/no-redeclare is acceptable (needed because type and const share the same name for backward compatibility)
• Check that factory functions maintain identical behavior to original class constructors

---

Link to Devin run: https://app.devin.ai/sessions/88c5d6992adf45b7b386051a3d7ed13c
Requested by: Steven Zhang (@joker23)

---

Note

Refactors datasource components to prefer interfaces and factory functions, with backward-compat shims.

• Replaces DataSourceState enum with a string union type and adds DataSourceState const for compatibility
• Converts DataSourceEventHandler, DataSourceStatusManager, and Requestor from classes to interfaces plus create*/makeRequestor factories using closure-based state
• Updates usages: state updates now pass string literals (e.g., "INITIALIZING", "CLOSED", "VALID"); imports switched to named imports
• Adjusts DataManager, BrowserDataManager, PollingProcessor, and StreamingProcessor to the new interfaces; no intended behavioral changes
• Updates tests to use new factories and named imports

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[devin-ai-integration]

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25394 bytes
Compressed size limit: 26000
Uncompressed size: 124693 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 171249 bytes
Compressed size limit: 200000
Uncompressed size: 797978 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 23264 bytes
Compressed size limit: 25000
Uncompressed size: 80869 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 18984 bytes
Compressed size limit: 20000
Uncompressed size: 98086 bytes

[devin-ai-integration]

Package Size Diff Report

@launchdarkly/js-client-sdk-common (size limit: 20,000 bytes brotli)

Metric	Main	PR	Diff
Raw	99,589 bytes	98,360 bytes	-1,229 bytes (-1.23%)
Brotli	19,322 bytes	18,992 bytes	-330 bytes (-1.70%)

@launchdarkly/js-client-sdk (size limit: 25,000 bytes brotli)

Metric	Main	PR	Diff
Raw	81,328 bytes	81,036 bytes	-292 bytes (-0.35%)
Brotli	23,330 bytes	23,301 bytes	-29 bytes (-0.12%)

The refactoring to interfaces + factory functions results in a small size reduction for both packages, with the sdk-client-common package seeing the most benefit (-1.70% brotli compressed). Both packages remain well under their size limits.

[joker23]

Can confirm ^ is correct