Limit parallel tool calls for spawn_agent tool.

[zikajk]

With >6 parallel spawn_agent tool calls some models can throw 503 for some.
E.g. I told Opus to run 10 GPT-5.2 subagents and two of them stopped responding in the middle.

I see two problems currently:
a) Error appears in the main chat window and not inside the subagent (I saw one error from OpenAI while two sub agents stopped responding and it wasn't clear those two failed - it was reviewing level design and reviewing game state).
@ericdallo - I would leave this one to you :-).

b) We can't control the maximum number of tool calls ( 6 by default might be reasonable for spawn_agent).

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[zikajk]

@ericdallo It looks like hard-limit is hard 😅 . It is kinda random so I think that letting sub-agent fail (a)) + hardening system instructions will be enough. The reason why Codex limits it to 6 is not related to the error we experience there at all.