feat: Propagate environment ID to evaluation hooks

[kinyoklion]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Implements the environmentId field of EvaluationSeriesContext from the hooks spec, which the OTel tracing hook uses for feature_flag.set.id. Mirrors launchdarkly/dotnet-core#81.

Describe the solution you've provided

EvaluationSeriesContext gains an optional environment_id, populated from the X-LD-EnvID response header sent by LaunchDarkly. Both data systems are supported, and each exposes it through a new DataSystem.environment_id property that the client reads when building the series context:

• FDv2 already parsed the header into Basis.environment_id / Update.environment_id but discarded it; FDv2 now latches the last non-None value (including from initializer error headers).
• FDv1 had no access to the header. Streaming reads it from the Start/Fault actions of the SSE client, polling reads it from the response headers, and both record it on DataSourceUpdateSinkImpl.

Sketch of the FDv1 path:

# streamingifisinstance(action, Start):
record_environment_id(self._data_source_update_sink, action.headers)
# polling
(all_data, headers) =self._get_all_data_with_headers()
record_environment_id(self._data_source_update_sink, headers)

Describe alternatives you've considered

Following the .NET implementation more literally, where the environment ID is stored as init metadata on the data store, would require optional extension interfaces on the public FeatureStore/DataSourceUpdateSink types. Instead, header handling stays inside the data sources and is surfaced by the data system, so externally implemented stores, sinks, feature requesters, and update processors continue to work unchanged (they simply report no environment ID).

Additional context

• No change is needed in launchdarkly-eventsource: it already exposes response headers on Start and Fault (unlike the .NET event source, which needed feat: Report event source headers on open. dotnet-eventsource#104).
• The async (experimental) client does not run hooks yet, so the async data source path is untouched.
• The environmentId support in the OTel tracing hook lives in python-server-sdk-otel and is a follow-up.

Link to Devin session: https://app.devin.ai/sessions/bfe54128e2804a96bb100e6120e9a3ef
Requested by: @kinyoklion

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