chore: aligning vue composables to trigger eval events

[joker23]

Note

Medium Risk
Changes when and how often flag evaluations run (including possible double eval on identify), which can affect analytics impressions and UI timing for all Vue variation composables.

Overview
useVariationCore no longer watches provider context / initializedState or batches updates through a flagChanged counter. Flag refs now re-evaluate from two explicit client hooks: change:<key> and onContextChange (settled start() and completed identify()), with cleanup on scope dispose.

Tests and makeMockClient were updated to assert subscription lifecycle (contextSubscriberCount), init-complete/failed paths via context notification (including SDK-2640 single eval on failure), and the new eval semantics: one eval when only the flag or only context changes, two when both fire on the same identify (documented as accepted React-parity cost, SDK-2194).

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[github-actions]

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

[github-actions]

@launchdarkly/browser size report
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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
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Uncompressed size: 114525 bytes

[joker23]

@cursor review