feat: Add hook-environment-id capability and test

[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

Ports #410 (v2) to v3. Companion SDK work: launchdarkly/python-server-sdk#484, launchdarkly/cpp-sdks#594.

Describe the solution you've provided

Same capability and test as #410, adapted to the v3 data system:

• the hook-environment-id capability, documented in docs/service_spec.md
• DataSystemOptionEnvironmentID(...), a top-level SDKDataSystemOption that makes every mock service of the data system (initializers and synchronizers, in all connection modes) send an X-LD-EnvID response header
• an evaluation-series test (hooks/evaluation/provides the environment ID) asserting evaluationSeriesContext.environmentId on afterEvaluation, run against the SDK's default data source plus an explicit polling subtest for client-side/server-side-polling SDKs

Unlike v2, where the option wrapped the single SDKDataSource handler, the header is applied in createEndpoints, so both FDv2 initializers and synchronizers report it — matching production, where the header is on every flag-delivery response.

Describe alternatives you've considered

Making the option per-connection-mode. Environment ID is a property of the environment, not of a connection mode, so a single top-level option keeps call sites simple and covers every mode.

Additional context

Verified against the FDv2 contract services of python-server-sdk (#484) and cpp-sdks (#594): hooks/evaluation/provides the environment ID passes for both, in default (streaming) and polling modes.

Track hooks (trackSeriesContext.environmentId) are documented but not yet asserted, same as in v2.

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

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Note

Overview
Adds the hook-environment-id capability so SDKs that read LaunchDarkly’s X-LD-EnvID from streaming/polling responses must surface it on hook series contexts (evaluationSeriesContext.environmentId, and trackSeriesContext when track hooks are supported). The harness documents this in service_spec.md and gates a new evaluation hook test on the capability.

Mock flag delivery now supports DataSystemOptionEnvironmentID(...), a top-level data-system option that wraps every initializer and synchronizer endpoint (all connection modes) to emit X-LD-EnvID, matching production flag-delivery responses in the v3 data system.

The test “provides the environment ID” evaluates a flag and asserts environmentId on the afterEvaluation hook payload, under the default data source and an explicit polling subtest for client-side / server-side-polling SDKs. Track-hook environmentId is documented but not asserted yet (same as v2).

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