[BUG][API]: Tool update resets visibility to public when field not provided

[KKNithin]

🐞 Bug Summary

While updating the tool if we don't provided the optional field visibility, then it considers it "public" by default and overrides the existing visibility of tool and makes it "public" visible.

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🧩 Affected Component

Select the area of the project impacted:

• mcpgateway - API
• mcpgateway - UI (admin panel)
• mcpgateway.wrapper - stdio wrapper
• Federation or Transports
• CLI, Makefiles, or shell scripts
• Container setup (Docker/Podman/Compose)
• Other (explain below)

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🔁 Steps to Reproduce

1. Have a tool which is visible "private" or "team"
2. update any field of tool but don't provide field "visibility"
3. update is successful but tool visibility is "public" now.

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🤔 Expected Behavior

If tool visibility is not provided then existing visibility should be taken as reference.

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[crivetimihai]

Thanks @KKNithin. The update endpoint should preserve existing visibility when the field is omitted. Prioritizing as MUST for RC2.