feat: stream vardata

[zhukaihan]

Summary

Adds vardata streaming feature.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[Copilot]

Pull Request Overview

This PR adds vardata streaming functionality to the Amplitude Experiment Javascript Client SDK. Key changes include:

• Introducing the SSEStream and related streaming components in the core package.
• Implementing the SdkStreamEvaluationApi for streaming variant data and updating tests accordingly.
• Integrating streaming functionality into the ExperimentClient with accompanying updates in configuration and updaters.

Reviewed Changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/experiment-core/test/utils.ts	Added a sleep utility for test timing.
packages/experiment-core/test/transport/stream.test.ts	New tests for verifying SSEStream connection, keepalive, and error handling.
packages/experiment-core/test/api/evaluation-stream-api.test.ts	New tests for stream-based variant evaluation; note duplicate test descriptions.
packages/experiment-core/src/transport/stream.ts	Added SSEStream implementation with keepalive and automatic reconnection logic.
packages/experiment-core/src/api/evaluation-stream-api.ts	Implements the streaming evaluation API using SSEStream.
packages/experiment-core/src/index.ts	Re-exports new streaming components.
packages/experiment-browser/test/util/misc.ts	Added sleep utility for browser tests.
packages/experiment-browser/test/client.test.ts	Updated tests to cover the new stream variants integration.
packages/experiment-browser/src/util/updaters.ts	Added updaters for handling streaming and fallback for variant updates.
packages/experiment-browser/src/transport/stream.ts	Introduces defaultSseProvider using EventSource.
packages/experiment-browser/src/experimentClient.ts	Updated ExperimentClient to support streamVariants with new updater integration.
packages/experiment-browser/src/config.ts	Added configuration options for streamVariants and its server URL.

Files not reviewed (1)

• packages/experiment-browser/package.json: Language not supported

Comments suppressed due to low confidence (1)

packages/experiment-core/test/api/evaluation-stream-api.test.ts:140

• Duplicate test description 'stream error stops api' detected in evaluation-stream-api tests; consider renaming one test to clarify different error scenarios.

test('stream error stops api', async () => {