Studio: add Anthropic and OpenAI prompt guards for disabled tools (#5674)

* Add Anthropic prompt guards for disabled tools

* fix: merge Anthropic tool guard into structured system prompts

* fix: scope Anthropic disabled-tool guard wording

* chore: adjust claude guard prompt

* chore: add openai to list of prompt guarded providers

* Studio: include web_fetch in the per-turn disabled-tool guard

Add webFetchEnabledForThisTurn alongside webSearchEnabledForThisTurn
and codeExecEnabledForThisTurn. Use it in the enabled_tools payload
so web_fetch follows the Search pill the same way web_search does,
and mention "web fetch" in the disabled-tool guard prose on providers
that ship the tool (Anthropic today; other providers stay inert via
providerSupportsBuiltinWebFetch).

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Co-authored-by: Roland Tannous <115670425+rolandtannous@users.noreply.github.com>
Co-authored-by: Daniel Han <danielhanchen@gmail.com>