feat: add client side bootstrap tests

[joker23]

This PR will add initial client side bootstrap contract tests to the test harness

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Overview
Adds client-side bootstrap contract testing to the SDK test harness: a new "bootstrap" capability, configuration wiring, and a full subtest suite that exercises pre-fetched allFlagsState-shaped flag data at SDK start.

The service spec now documents the capability and the clientSide.bootstrap object ($flagsState, $valid, replacement by the first live synchronizer payload). SDKConfigClientSideParams gains a bootstrap field with JSON serialization tests, and the client-side suite registers a bootstrap test group gated on CapabilityBootstrap.

New harness tests build bootstrap payloads via bootstrapPayloadBuilder, use withOfflineDataSystem() / live streaming setups, and cover immediate evaluation without network, initialization without waiting on the stream, metadata for analytics (not in all-flags), prerequisite events, null flag values → defaults, $valid: false still ingested, legacy payloads without $flagsState, and replace-not-merge when live data arrives.

Also fixes a read lock leak in mockld.StreamingService.makeXferFull when initial data is not FDv2 (adds RUnlock before early return).

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[joker23]

@cursor review

[devin-ai-integration]

✅ Devin Review: No Issues Found

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