feat: Add Node platform abstraction for node-client

[joker23]

Summary

• Adds the Node platform abstraction layer for @launchdarkly/node-client-sdk: HeaderWrapper, NodeCrypto, NodeEncoding, NodeInfo, NodePlatform, NodeRequests, NodeResponse, NodeStorage. These implement the Platform contract from @launchdarkly/js-client-sdk-common.
• Adds runtime dependencies the platform code imports: @launchdarkly/js-client-sdk-common, https-proxy-agent, launchdarkly-eventsource.
• Modernizes packages/sdk/node-client/tsconfig.json to module: ESNext + moduleResolution: bundler (tsup handles emit), adds esModuleInterop, types: ["jest", "node"], skipLibCheck. Matches the pattern used by packages/sdk/server-ai.
• Part of the SDK-2195 stacked migration of launchdarkly-node-client-sdk into js-core. Follows PR chore: scaffolding node-client-sdk migration #1352 (scaffold).

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Note

Medium Risk
New networking (TLS/proxy), persistent cache I/O, and crypto primitives affect how the SDK talks to LaunchDarkly and stores user data; behavior is heavily tested but not yet covered by contract tests.

Overview
Adds the Node Platform implementation for @launchdarkly/node-client-sdk so the shared client can run on Node: crypto/encoding/info, HTTP/SSE via NodeRequests (proxy/TLS agents, optional POST gzip, launchdarkly-eventsource), fetch-like responses/headers, and file-backed NodeStorage with a process singleton and error-logging wrapper in NodePlatform.

Introduces NodeOptions (proxy, TLS, compression, cache path, logger), wires runtime deps (js-client-sdk-common, https-proxy-agent, launchdarkly-eventsource), and adds Jest coverage plus a GitHub Actions matrix (Node 18/22) for the workspace. TypeScript moves to ESNext / bundler resolution for tsup; release-please bumps version in NodeInfo.ts.

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26389 bytes
Compressed size limit: 29000
Uncompressed size: 129320 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31894 bytes
Compressed size limit: 34000
Uncompressed size: 113696 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38516 bytes
Compressed size limit: 39000
Uncompressed size: 211129 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179574 bytes
Compressed size limit: 200000
Uncompressed size: 830875 bytes

[joker23]

@cursor review

[joker23]

@cursor review

[joker23]

Old sdk used a package for storage and we implement it here. A lot of this logic is ported from what we have in the electron sdk for reference.

[joker23]

I think we should allow for overriding the storage. Similar to what we do in the react-native sdk