fix(llm): fail closed on malformed native tool calls

[canblmz1]

Summary

Prevents native OpenAI-compatible tool calls from executing when their arguments are malformed or when a tool-call stream ends without a confirmed terminal signal.

Root cause

Two behaviors combined:

• malformed non-empty tool arguments were silently parsed as {} after JSON.parse failed
• a tool-call stream ending by bare EOF with neither finish_reason nor [DONE] was treated like a confirmed completion

That allowed malformed or unconfirmed tool arguments to reach the normal execution path.

Fix

• malformed non-empty tool arguments now surface through the existing parse failure / repair path instead of becoming {}
• the stream consumer records whether a terminal signal was actually observed
• pending tool calls whose stream ends without a terminal signal route through the existing truncated/model-failure path before dispatch

Legitimate zero-argument calls remain supported.

A provider that sends an explicit finish_reason without [DONE] remains accepted, explicit finish_reason: "length" behavior is unchanged, and plain-text-only responses are unaffected.

Validation

Execution-level regression coverage using the real provider, step executor, ToolRegistry, and instrumented no-op tools:

• healthy valid tool call -> executes once
• malformed args + clean terminal -> 0 executions
• malformed args + abrupt EOF -> 0
• container-level truncation + EOF -> 0
• syntactically complete args + ambiguous EOF -> 0
• explicit finish_reason: "length" -> 0
• parallel complete + truncated under ambiguous EOF -> 0 / 0
• stream read error -> 0

Also verified:

• explicit finish_reason without [DONE] remains accepted
• plain-text bare-EOF behavior remains unchanged

Tests

• npm run lint (typecheck): clean
• npm run build: clean
• Targeted suites (openai provider/stream-consumer/tool-call-adapter, step-executor, reliability, parallel-tool-calls integration, plus the new execution-integrity suite): 135/135 passed
• Full suite: 3983 passed, 20 failed, 5 skipped (4008 total)
• The same 20 failures reproduce identically on unmodified current main (same test names, same files) — confirmed by running the full suite on both in the same session. No new failure names introduced by this change.