docs: clarify API tool and MCP support

[zuyua9]

Summary

• clarify that direct API calls can use Open WebUI-managed tools when the model is configured for native tool calling
• document how request-level features map to direct API tool toggles
• note the OAuth MCP limitation for unattended API clients and link the guidance back to issue issue: (Docs) Cannot find in API documentation whether (or not) MCP tools can be enabled in API calls #645

Testing

• arch -x86_64 /usr/bin/git -C /Users/ming/code/open-webui-docs-zuyua9 diff --check

[pr-validator-bot]

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[Classic298]

warning block "tool calling will not work if .."

[Classic298]

'typically'?