feat: Add eventSource factory and usePost options to the client-side SDK common

[joker23]

Summary

Adds two pieces to the client-side SDK common in support of customizable streaming transports:

• LDEventSourceFactory: a public contract for supplying a custom EventSource implementation (createEventSource(url, initDict) plus a capabilities declaration -- customMethod, headers, readTimeout). The SDK consults the declared capabilities before using a transport feature; a factory that declares nothing is treated as supporting nothing.

• usePost option (early access, FDv2 dataSystem only): when enabled, streaming and polling flag requests use POST with the evaluation context in the request body instead of GET with the context in the URL path, keeping the context out of URL-based request logs. Streaming POST requires an EventSource whose factory declares customMethod; because there is no fallback on that path, the client throws at construction time if usePost is set without a compatible EventSource. useReport has no effect under FDv2 (usePost is its equivalent there) -- it is ignored with a warning rather than silently changing behavior.

Plumbing: the FDv2 streaming base accepts a method and body, and the source factory provider selects the POST paths and passes the context body through to both the stream and its polling requestor. Covered by new construction-guard, provider, and streaming-base tests.

🤖 Generated with Claude Code

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Note

Overview
Adds LDEventSourceFactory as a public contract for custom streaming transports, including optional capabilities (notably customMethod) that the SDK consults before using non-default HTTP methods on streams.

Introduces the early-access usePost option for FDv2 when dataSystem is enabled: polling and streaming can send the evaluation context in a POST body instead of a GET URL. Streaming POST is used only when the platform EventSource reports customMethod; otherwise the client logs a warning and streaming stays on GET while polling still uses POST. useReport is documented and enforced as ineffective under dataSystem (warning + forced off; use usePost instead).

Wiring runs through configuration validation, SourceFactoryProvider (path selection and method/body on StreamingFDv2Base), and client construction checks in LDClientImpl. Tests cover provider behavior, POST stream init, and config warnings.

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[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 size report
This is the brotli compressed size of the ESM build.
Compressed size: 32886 bytes
Compressed size limit: 34000
Uncompressed size: 117900 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25687 bytes
Compressed size limit: 44000
Uncompressed size: 166720 bytes

[joker23]

moved this back to draft so I can sneak in another layer below this one.