plugin-flow-builder: move contentFilter from action to each content

[Iru89]

Description

This PR moves Flow Builder content filtering from the action-level content list into each content field processing lifecycle. It also ensures AI agent responses are filtered after the response is resolved and before their messages are converted into Botonic JSX elements.

Context

Flow Builder content filters were previously applied once in FlowBuilderAction before processing the full list of contents. That made AI agent responses harder to align with the rest of the content lifecycle because their generated messages were converted to JSX during response resolution, before the content processing step could apply filters consistently.

Approach taken / Explain the design

• Added ContentFieldsBase.filterContent, which retrieves the Flow Builder plugin content filters and runs them through ContentFilterExecutor for a single FlowContent instance.
• Removed the global filterContents helper and its usage from FlowBuilderAction, so content processing owns when filtering happens.
• Applied filterContent inside each content field processContent implementation before tracking, handoff, bot actions, or rendering-related work.
• Updated FlowAiAgent so response resolution only stores the AI agent messages, then processContent filters the content and converts the final messages to Botonic JSX elements.

Testing

The pull request...

• doesn't need tests because this change refactors when existing content filters are executed without changing the filter API or adding new user-facing behavior.

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