feat: Migrate the node-family SDKs to @launchdarkly/eventsource through the platform fetch

[joker23]

Summary

Moves the SSE streaming transport of the node-family SDKs (server-node, node-client, electron) onto the platform Requests.fetch, using the optional streaming members added in #2092. Each SDK's createEventSource now injects (url, init) => requests.fetch(url, { ...init, streaming: true }) into @launchdarkly/eventsource, and the parallel fetch-adapter modules are deleted. The external launchdarkly-eventsource dependency is dropped.

Per package: the streaming branch sends no accept-encoding, applies no request timeout, sets setNoDelay(true), honors the abort signal, and never follows redirects; a new non-buffering streaming response class exposes the body reader; HeaderWrapper gains forEach. The buffered fetch path is unchanged. server-node applies per-request TLS parameters only when the SDK built the agent; node-client builds its agent from tlsParams with no proxy support; electron uses the machine's own network configuration.

Behavior notes, carried over from the adapter design:

• Streaming connections verify TLS by default; a stream served behind a private CA needs tlsParams.
• Streaming requests never follow redirects.

All legacy adapter test scenarios are ported to the new seam. Contract tests pass for all three SDKs.

[joker23]

move this to draft so we can explore ways to extend Platform.Request to support streaming.

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32662 bytes
Compressed size limit: 34000
Uncompressed size: 117055 bytes

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 29603 bytes
Compressed size limit: 30000
Uncompressed size: 142116 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25437 bytes
Compressed size limit: 44000
Uncompressed size: 165420 bytes