[BUG] Pydantic ValidationError when sending message after editing markdown table in Canvas

[epamLDadayan]

Description:
After editing a markdown table in Canvas, the next chat message fails because AIMessage.content is serialized as a list (with dict segments and a None) instead of a plain string. This triggers a pydantic_core ValidationError.

Steps to reproduce:

1. In Chat, ask the model to “create a sample markdown table.”
2. Open the table in Canvas and edit any cell.
3. Send a new message in the chat.

Actual Result:
Message fails with:

• pydantic_core.ValidationError for AIMessage
• content.str: “Input should be a valid string”
• content.list[…].2.str/dict: “Input should be a valid string/dictionary”, input_value=None

Expected Result:
Message sends successfully. Canvas edits are serialized back to valid plain markdown (string), with no validation errors.

Note: Reproducable in Stage and NEXT envs.

pydantic_core._pydantic_core.ValidationError: 3 validation errors for AIMessage
content.str
Input should be a valid string [type=string_type, input_value=[{'type': 'text', 'text':...perly formatted table'}], input_type=list]
For further information visit https://errors.pydantic.dev/2.10/v/string_type
content.list[union[str,dict[any,any]]].2.str
Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
For further information visit https://errors.pydantic.dev/2.10/v/string_type
content.list[union[str,dict[any,any]]].2.dict[any,any]
Input should be a valid dictionary [type=dict_type, input_value=None, input_type=NoneType]
For further information visit https://errors.pydantic.dev/2.10/v/dict_type

[epamLDadayan]

@daniilmotsniy seems we had this issue and it was fixed by @plageat but only deployed on DEV env:
#2069

[daniilmotsniy]

@epamLDadayan fix verified, deployed on stage

[epamLDadayan]

Verified on Stage env, @daniilmotsniy thanks