chore: Support headers when streaming

[jsonbailey]

Note

Adds header-aware streaming behavior and tests.

• In StreamingDataSource, handle on_connect headers to set environment_id and immediately trigger FDv1 fallback when x-launchdarkly-fd-fallback is true; refine on_error to read headers for environment_id and fallback, and propagate environment_id in updates
• Upgrade ld-eventsource to 2.5.0 in launchdarkly-server-sdk.gemspec
• Add specs spec/impl/data_system/streaming_headers_spec.rb covering header extraction and FDv1 fallback on connect/error

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