Restore instructions from transcript

[finnvoor]

in order to 100% restore a session from a transcript (necessary to maintain cache for some models), the system instructions should be loaded from the transcript. Right now they are nil when using the transcript init of LangaugeModelSession.

• Restore session-level instructions when constructing a LanguageModelSession from a transcript with a leading instructions entry
• Recover the instruction text directly from the single text segment emitted by LanguageModelSession, preserving it byte-for-byte
• Add one focused regression test for transcript-based instruction restoration

[pcuenca]

Thanks a lot @finnvoor, looks good to me.