[BUG] Tool message validation is unclear to end users

[RogerHYang]

always reverts to the null string

Screen.Recording.2025-02-10.at.6.15.01.AM.mov

[dosubot]

I found 1 similar issue that might be helpful:

• [playground][ui] persist message content when switching between tool calls and string content]([playground][ui] persist message content when switching between tool calls and string content #5410) which was resolved by a pull request that updated error handling in the playground.

[cephalization]

The input for a tool role message must be valid JSON. Therefore it must look something like

"good"
{ "good": "value" }
["good"]

We should make this clearer via labeling or similar