test(e2e): restore Google tool-use and multi-plugin hook coverage

[chrisnestrud]

Summary

The full tagged E2E suite has two failures unrelated to the Responses reasoning lifecycle PR:

• TestGoogleGenaiE2E_ToolUseRoundTrip: expected function_call output item is absent.
• TestPluginHooks_MultiplePlugins: the second plugin MutateCoreRequest hook is not called.

Reproduction

CGO_ENABLED=0 go test -tags=e2e ./internal/e2e

Scope

The focused Responses/Anthropic reasoning replay E2E passes. This follow-up tracks restoring the remaining tagged E2E suite.