fix(execution): treat empty initial turns as successful terminal responses (light-tested)

[1688mengdie]

Closes #2658

Summary

A turn whose first round carries only system-injection user messages (no real user content) is finalized locally without a model request; the success whitelist only accepts max_rounds/repeated_tool_failures, so this terminal reason classifies as failure. Add empty_initial_turn to the whitelist so locally synthesized terminal turns settle as success like the other no-model-request reasons.

Note: no production path emits empty_initial_turn at this baseline (grep zero hits across the tree at 32f2427); this is a defensive alignment so the reason settles correctly once an empty-input first-round guard is introduced upstream.

Changes

• src/crates/assembly/core/src/agentic/execution/execution_engine.rs: the success whitelist (:4983) gains "empty_initial_turn" next to "max_rounds" | "repeated_tool_failures" (single line).
• Commit: 5e3c861 — fix(execution): treat empty initial turns as successful terminal responses

Testing

• cargo check --locked -p bitfun-core --jobs 4 → exit 0
• cargo test --locked -p bitfun-core --lib --features agent-runtime --jobs 4 execution_engine → 48 passed, 0 failed (the agentic tree is behind the agent-runtime feature gate since the crate has default = [])

Environment

All platforms; baseline 32f2427.

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AI-assisted change, reviewed and verified as described above (light-tested locally).