fix(multishot): avoid empty driver messages

[drewstone]

Summary

• represent tool-only assistant turns as concise non-empty tool-call summaries in the multishot driver transcript
• skip blank transcript rows before calling the persona driver
• bump agent-eval / Python client fallback version to 0.96.3

Verification

• pnpm vitest run tests/multishot/multishot.test.ts — 8/8 passed
• pnpm typecheck — passed
• pnpm test — 2529 passed, 2 skipped
• python3 -m py_compile clients/python/src/agent_eval_rpc/init.py — passed
• NODE_OPTIONS=--max-old-space-size=8192 pnpm build — passed
• pnpm verify:package — passed
• git diff --check — passed
• git merge-tree --write-tree origin/main HEAD — passed

Why

Tax Agent state multishot runs were failing with router 400 Invalid request: text content is empty after tool-only assistant turns. This belongs in the shared multishot runner, not as a Tax-local workaround.

[tangletools]

✅ Auto-approved PR — 6ce38b09

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_{tangletools · auto-approval · reason: blanket_auto_approve · 2026-06-22T17:36:39Z}