fix: include experiment key on local-eval exposure events

[kyeh-amp]

Summary

• Local evaluation via evaluateV2 was emitting exposure events that included the variant metadata blob (containing metadata.experimentKey) but never lifted the experiment key onto a top-level event property. Custom reports that key off [Experiment] Experiment Key therefore saw no value for customers on local evaluation.
• Fix: in toExposureEvents, when variant.metadata?.experimentKey is truthy, set event_properties['[Experiment] Experiment Key']. Additive and backwards-compatible — the existing metadata blob, [Experiment] Flag Key, [Experiment] Variant, user properties, and insert_id are unchanged.
• Mirrors the equivalent behavior in the browser SDK (experiment-js-client/.../experimentClient.ts sets exposure.experiment_key = sourceVariant.variant?.expKey). Property name uses the bracketed-namespace convention already established in this file ([Experiment] Flag Key, [Experiment] Variant) rather than the browser SDK's snake_case, since the browser SDK emits via a typed Exposure object that gets translated downstream, whereas this file emits Amplitude events directly.

Test plan

• Added with_experiment_key fixture to the existing toExposureEvents test in packages/node/test/local/exposure/exposure-service.test.ts. Asserts the new property equals 'exp-1' for that flag and is undefined for all other flags in the same exposure.
• Updated the canonicalization expectation in the same test to account for the new fixture entry (with_experiment_key treatment appended in alphabetical order).
• yarn workspace @amplitude/experiment-node-server test — 208 passed, 1 todo, 0 failures.
• yarn workspace @amplitude/experiment-node-server lint — 0 errors (19 pre-existing warnings, none from this change).
• yarn workspace @amplitude/experiment-node-server build — clean.

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Note

Low Risk
Low risk additive change that only adds an extra event property when variant.metadata.experimentKey is present; main risk is minor downstream analytics/reporting impact from a new field.

Overview
Local-eval exposure events now lift variant.metadata.experimentKey into a top-level event property (event_properties['[Experiment] Experiment Key']) when present, aligning exposure reporting with other SDKs.

Tests extend the toExposureEvents fixture to include a flag with an experimentKey, assert the new property is set only for that flag, and update expected event count and insert_id canonicalization accordingly.

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