Fix ABTesting UpdateExperiments overflow policy binding

[AdamEssenmacher]

Summary

• correct Firebase.ABTesting.ExperimentController.UpdateExperiments to use the native overflow-policy enum instead of NSObject
• add the missing ExperimentPayloadExperimentOverflowPolicy enum with the values defined in the Firebase ABTesting source
• add a focused local E2E case that verifies the managed signature and a representative native call path
• harden the local E2E runner so case-injected package references and packed local packages are restored deterministically

Validation

• dotnet pack source/Firebase/ABTesting/ABTesting.csproj --configuration Release --output output
• tools/e2e/run-firebase-foundation.sh --package-dir output --configuration Debug --runtime-drift-case abtesting-updateexperiments
• tools/e2e/run-firebase-foundation.sh --package-dir output --configuration Debug

Notes

• This is a real API-fidelity fix, not a runtime-failure remediation. The old NSObject parameter was too broad, but the selector still completed at runtime.
• The runner changes are included because this case exposed two local validation issues: stale restore evaluation for case-injected package references and stale package extraction when reusing the same local package version.

[AdamEssenmacher]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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