feat: commonizes several FDv2 related types in Server

[tanderson-ld]

This PR is the third installment of commonizing FDv2 related types. There should be no behavioral changes in this PR.

---

Note

Medium Risk
Broad refactor across data-source and data-store update paths to switch to shared com.launchdarkly.sdk.fdv2 types plus an internal dependency bump, which could surface integration/compatibility issues despite intended no behavior change.

Overview
Commonizes FDv2 model types by switching server SDK code from internal/DataStoreTypes FDv2 classes to shared com.launchdarkly.sdk.fdv2 types (Selector, ChangeSet, ChangeSetType, SourceResultType, SourceSignal) and deleting DataStoreTypes.ChangeSet/ChangeSetType.

Updates all affected interfaces/implementations and adapters (TransactionalDataStore, TransactionalDataSourceUpdateSink, FDv2SourceResult, polling/streaming/file/test sources) to use the new ChangeSet generic shape, and adjusts minor edge-case handling (e.g., sortChangeset null-data guard; polling 304 now emits an empty data list).

Build changes bump launchdarkly-java-sdk-internal to 1.9.0 and include com.launchdarkly.sdk.fdv2 in the package list used for OSGi/shading; tests are updated accordingly.

^{Written by Cursor Bugbot for commit aeda35a. This will update automatically on new commits. Configure here.}

BEGIN_COMMIT_OVERRIDE
feat: commonizes several FDv2 related types
END_COMMIT_OVERRIDE

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}