fix api key inconsistency and cleanup in live api quickstart

[knQzx]

Summary

• fixed undefined GOOGLE_API_KEY variable — now reads from os.environ like the rest of the notebook
• added a warning when native-audio model is selected since it doesn't support TEXT response modality
• removed redundant client/model redefinition in the resumable sessions section

Test plan

• verified notebook cells reference the correct api key variable
• verified model guard prints warning for native-audio models
• verified resumable sessions section works without redundant client init

fixes #948

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[knQzx]

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