Expand smoke tests for transcript payload variants

[zp6]

Changes

Expands scripts/smoke_test.py as requested in #2 with new test cases:

Transcript payload variants

• Nested assistant content in list format (content: [{type: text, text: ...}])
• Nested content in dict format (content: {type: text, text: ...})
• Assistant content nested under message key

Clarifying question detection

• Transcript with clarifying question response should pass (non-assertive)

Caveat detection edge cases

• best-effort wording in transcript
• unable to verify wording in transcript

Malformed/empty transcript handling

• Empty lines and non-JSON lines should fail open, not crash
• Transcript with no assistant messages
• Non-existent transcript file path

Also adds helper functions (fresh_session, run_gate, run_track, run_stop, write_transcript) to reduce boilerplate.

Fixes #2

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Wallet: zp6