fix: browser logging for non-existing flags

[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-1736

Describe the solution you've provided

• When we call variation on a non-existent flag, we now should only get a warning message and the default flag value will be returned
• We will no longer emit an error to the client side emitter, but we will still return an error detail as well as send an error event to LD

Additional context

This behavior should now be consistent with what we have in v3 (https://github.com/launchdarkly/js-sdk-common/blob/main/src/index.js#L339). The reason for this change is that there is a valid chance that a missing flag is not considered a "error" level event on the application as developers can handle these cases with defaults (and more specialized handling by looking at the variation detail).

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Note

Aligns unknown-flag behavior with v3 by de-escalating from client error events to warnings.

• In LDClientImpl._variationInternal, replace emitter error with logger.warn for unknown flags; still send unknownFlagEvent and return createErrorEvaluationDetail(FLAG_NOT_FOUND, ...).
• Update tests in LDClientImpl.variation.test.ts to expect a warning and default value for variation, and an error detail for variationDetail.

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@launchdarkly/js-sdk-common size report
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[github-actions]

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[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
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[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 19354 bytes
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Uncompressed size: 99752 bytes