feat: Add an opt-in fetch-based EventSource entry to the browser SDK

[joker23]

Summary

Adds an opt-in fetch()-based EventSource to the browser SDK behind its own @launchdarkly/js-client-sdk/fetch-eventsource entry point, so neither the implementation nor its dependency reaches the default bundle. The native browser EventSource remains the default. Includes the eventSource factory option plumbing, an example app, and bundle-size coverage.

On top of that, the browser contract test entity declares the client-use-post capability and injects the fetch-based EventSource whenever the harness configures usePost or useReport, so the FDv2 POST variants and the FDv1 REPORT streaming variants now run. Stale suppressions removed accordingly (6 REPORT lines from suppressions.txt, 22 dead lines from suppressions_datamode_changes.txt); verified locally with no-suppression harness runs (FDv1: 823 ran, FDv2: 795 ran incl. 20 POST variants, all critical tests passed).

Replaces #2034 and #2048, which GitHub auto-marked merged during a stack reorder; their bot-review history lives there.

[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: 25686 bytes
Compressed size limit: 44000
Uncompressed size: 166718 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32947 bytes
Compressed size limit: 34000
Uncompressed size: 118122 bytes