fix(sanitizer): drop tool_calls key when all calls deduped, not set empty array

[yanjingr119]

Problem

When sanitize_api_messages deduplicates tool_call_ids, if all calls in a message are duplicates, the current code sets tool_calls: [] on the message dict. Strict providers (DeepSeek v4) reject this with HTTP 400:

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

The existing pre-API chokepoint (lines 2738-2770) that drops empty/invalid tool_calls arrays runs before the dedup pass and cannot see arrays emptied here.

Root Cause

# Current code (line 2896):
if len(kept_tcs) != len(msg.get("tool_calls") or []):
    msg = {**msg, "tool_calls": kept_tcs}  # ❌ kept_tcs=[], produces tool_calls:[]

Fix

When kept_tcs is empty (all calls were duplicates), drop the tool_calls key entirely instead of preserving an empty array. If dropping the key would leave the message body empty (edge case: host-fed/merged histories from WebUI sidecar), supply a placeholder content string.

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"}
        if not str(msg.get("content") or "").strip():
            msg = {**msg, "content": "(duplicate tool call removed)"}

Testing

• Reproduced: DeepSeek v4 HTTP 400 when WebUI sidecar + state.db merge produces duplicate tool_call_ids that all get deduped
• Verified: same payload passes after fix (key dropped, no empty array)

[alt-glitch]

This was generated by AI during triage.

Duplicate of #64345. It already implements the same dedup-output drop-key repair with focused regression coverage; this patch's placeholder-content edge case is the only material delta.

[yanjingr119]

Duplicate of #64345 which covers the same fix. The empty-content placeholder edge case in this PR is the only delta and is a rare scenario — closing in favor of #64345. Thanks for the review!