chore: aligning vue composables to trigger eval events

[joker23]

Note

Medium Risk
Changes when flag variation runs (init, identify, analytics impressions), including a deliberate double-eval edge case; scope is limited to the Vue SDK composable layer.

Overview
useVariationCore no longer batches re-evaluation through a Vue watch on provider context, initializedState, and a flagChanged counter. It now wires change:<key> and client.onContextChange directly to the same update handler, and only watches the flag key to swap listeners.

That aligns readiness and identify() updates with the client’s context notifications (including failed start() via a single context callback, avoiding a separate init-status path). When one identify() updates both context and the watched flag, two evaluations are possible and tests document that as accepted React-parity behavior.

Tests and makeMockClient add contextSubscriberCount and expand coverage for subscribe/unsubscribe, init complete/fail, and single- vs dual-trigger eval counts.

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[github-actions]

@launchdarkly/js-sdk-common size report
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Compressed size: 26360 bytes
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Uncompressed size: 129188 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179691 bytes
Compressed size limit: 200000
Uncompressed size: 831704 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 39108 bytes
Compressed size limit: 39300
Uncompressed size: 213642 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32077 bytes
Compressed size limit: 34000
Uncompressed size: 114525 bytes

[joker23]

@cursor review