🛡️ Sentinel: [HIGH] Fix XSS vulnerabilities in event previews

[kevinaboos]

🚨 Severity: HIGH
💡 Vulnerability: User-controlled fields in event previews (such as room aliases, custom guest access rules, and custom join rules) were injected directly into the HTML preview strings without escaping, creating a Cross-Site Scripting (XSS) vulnerability.
🎯 Impact: An attacker could inject malicious HTML or disrupt the UI layout of users viewing the event previews, potentially executing arbitrary actions within the client context.
🔧 Fix: Wrapped the user-controlled inputs with htmlize::escape_text inside text_preview_of_other_state in src/event_preview.rs whenever format_as_html is true. Used Cow::Owned(htmlize::escape_text(custom.as_str()).into_owned()) to maintain type compatibility in match arms.
✅ Verification: Ran cargo check and cargo test event_preview to ensure the escaping logic compiles correctly and resolves the type mismatch errors.

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PR created automatically by Jules for task 175853728165591768 started by @kevinaboos

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