fix(agent): drop tool_calls key when dedup empties the array

[dahux-net]

Why

Long-lived Hermes sessions intermittently 400 on strict providers (DeepSeek):

HTTP 400: Invalid 'messages[N].tool_calls': empty array. Expected an array
with minimum length 1, but got an empty array instead.

The pre-API sanitizer (sanitize_api_messages) drops tool_calls: [] on
assistant messages (#58755), but its own tool_call_id dedup pass can
re-create it: when an assistant message's calls are ALL duplicates of ids
already seen earlier in the transcript (crash/resume glitch, retry, or a
compression window re-emitting a tool result), kept_tcs ends up empty and
the pass writes msg["tool_calls"] = [] back onto the message — after the
empty-array pass already ran.

DeepSeek rejects the empty array outright, the request fails, and because
the fallback chain's first leg resolves to the same backend, the failure
falls through to the next provider. The poisoned session 400s the same way
every subsequent turn.

Fix

When dedup empties the array, drop the tool_calls key entirely — the same
semantics the empty-array pass already applies (empty array ≡ no tool
calls):

if len(kept_tcs) != len(msg.get("tool_calls") or []):
    if kept_tcs:
        msg = {**msg, "tool_calls": kept_tcs}
    else:
        msg = {k: v for k, v in msg.items() if k != "tool_calls"}

The pre-existing empty-array case still cleans up as before; healthy
transcripts are untouched; partial dedup keeps the surviving (first)
calls.

Test

tests/run_agent/test_sanitize_dedup_empty_tool_calls.py — 3 tests:

• all-duplicates block yields a plain assistant message, never []
  (this one fails on the old code)
• unique-id transcript preserved
• partial duplicates keep the first occurrence

Verified: scripts/run_tests.sh tests/run_agent/test_sanitize_dedup_empty_tool_calls.py tests/run_agent/test_message_sequence_repair.py tests/run_agent/test_tool_call_args_sanitizer.py tests/run_agent/test_thinking_only_sanitizer.py -q — all pass.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #64345: it implements the same all-deduped tool_calls cleanup in sanitize_api_messages, with the same strict-provider empty-array failure mechanism.

[dahux-net]

Closing as redundant: upstream's general empty/invalid tool_calls sanitizer (agent/agent_runtime_helpers.py, follow-up to #58755) covers this case class — it drops the tool_calls key for any empty or non-list value, including the dedup-empties case this PR addressed. Verified present in v0.20.0. Thanks for reviewing.