FIX fail closed on realtime audio swap errors

[Roman Lutz (romanlutz)]

Why

When a realtime request converter inserts converted audio, failure to delete the original committed audio was logged and ignored. The session then requested a response with both raw and converted audio in context, silently defeating the converter.

This is a follow-up to #2463, which covers audio-source iterator failure earlier in the streaming lifecycle. It does not exercise the committed-audio replacement path.

What changed

• Propagate raw-item deletion failures instead of continuing to response.create.
• Preserve the successful-path ordering: insert converted audio, then delete raw audio.
• Add regression coverage proving deletion failure prevents response creation, audio saving, and message persistence.

Validation

• 33 focused realtime streaming-session tests passed.
• 1,163 prompt-target tests passed, 30 skipped.
• Ruff, formatting, changed-file typing, compilation, lock, and diff checks passed.