Remove realtime initial ack startup gate

[AdirAmsalem]

Summary

• Bundle the initial realtime prompt/image (including the null-image bootstrap) into livekit_join.initial_state.
• Stop blocking LiveKit room connect / track publishing on the initial prompt/image ack.
• Report failed or timed-out initial-state acks through the existing async onError callback while keeping normal post-connect setPrompt/setImage calls blocking.

Context

• Mirrors the startup behavior from [codex] Remove realtime initial state startup gate sdk#166.
• No public issue/trigger is linked for this public repo.

Verification

• ./scripts/format.sh
• git diff --check
• cmake -B build -S . -G Ninja -DCMAKE_BUILD_TYPE=Release -DDECART_BUILD_EXAMPLES=ON -DDECART_BUILD_TESTS=ON && cmake --build build && ctest --test-dir build --output-on-failure -C Release
• codex exec --dangerously-bypass-approvals-and-sandbox "Review the uncommitted diff..."
• GitHub CI: ubuntu, macOS, Windows, and clang-format all pass.