fix(gen-ai): Gracefully handle malformed JSON in AI message parsing

[sentry]

This PR addresses the issue of SyntaxError exceptions occurring when parsing AI messages, specifically in the transformPartsMessages function within the AI Agents insight UI.

The root causes identified were:

1. PII-filtered content: Relay replaces sensitive data with the literal string [Filtered], which is not valid JSON.
2. Malformed JSON escape sequences: Some AI SDKs or user-provided content can contain invalid escape sequences (e.g., \p) that cause JSON.parse to fail.

Changes Made:

• parseJsonWithFix (utils.tsx): Modified to gracefully handle the above failure modes. It now explicitly checks for [Filtered] content and returns null for parsed data, and also wraps the fixJson fallback in an additional try/catch block to prevent unrepairable malformed JSON from throwing. In both cases, it returns {parsed: null, fixedInvalidJson: true}.
• transformPartsMessages (aiInput.tsx): Exported this function for improved testability.
• utils.spec.tsx: Updated existing tests for parseJsonWithFix to accurately reflect the new graceful handling of [Filtered] content and bad escape sequences.
• aiInput.spec.tsx (new file): Added a comprehensive test suite for transformPartsMessages, covering valid JSON transformations, passthrough of non-parts messages, and specific test cases for the [Filtered] placeholder and malformed escape sequences to ensure the fixes work as expected.

These changes prevent the UI from crashing due to unparseable AI message data, providing a more robust user experience.

Closes TET-2021

[linear]

TET-2021 Conversations page does not handle [Filtered] messages

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Filtered check skips recoverable JSON repair

Medium Severity

parseJsonWithFix now returns early whenever the raw string contains [Filtered] after a parse failure, so it never attempts fixJson. This can drop recoverable payloads where parsing failed for truncation or minor corruption unrelated to the placeholder, causing callers to lose structured message data and fall back to raw/empty output.