fix: drop a tool call that generation never finished

[Javinator9889]

Fixes #741.

When neither </tool_call> nor </function> is present, generation was cut off
inside the call — normally by the output limit while writing a large argument.
The parser took the remainder of the text as the block and emitted a
complete-looking tool call built from partial input:

} else {
    block_end = response_text.length();   // fabricates a complete call
}

The client cannot detect this, because the arguments JSON is well formed — it
is rebuilt from the parsed parameters, not copied from the raw text. A shell
heredoc cut mid-body becomes an unterminated cat <<EOF.

This drops the block instead, so no tool call is emitted. The response still
carries finish_reason: "length", which is the honest signal that the client
should retry with a larger budget. The </function> fallback is kept: a block
missing only the outer tag is complete.

Applied to the three models that share the pattern: qwen3_5vl,
qwen3_6_moe, qwen3_5_omni.

Verified on qwen3.5:9b, 300-line heredoc at max_tokens=400:

	before (92f3f13)	after
truncated	tool_calls: true, 1580 chars of arguments, no closing EOF	tool_calls: false, finish_reason: length
complete	finish_reason: tool_calls, {"command":"echo hello"}	unchanged