move content type fallback text to ffis

[mchenani]

Expose leave request fallback text via bindings_ffi::mls::leave_request_fallback and set xmtp_content_types::leave_request::LeaveRequestCodec::encode to populate EncodedContent.fallback

Add LeaveRequestCodec::FALLBACK_TEXT and use it in encode to set EncodedContent.fallback; add a UniFFI function leave_request_fallback() that returns the fallback text via FFI in bindings_ffi/src/mls.rs and xmtp_content_types/src/leave_request.rs.

📍Where to Start

Start with the UniFFI export leave_request_fallback() in bindings_ffi/src/mls.rs, then review LeaveRequestCodec::FALLBACK_TEXT and its use in encode in xmtp_content_types/src/leave_request.rs.

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Macroscope summarized 932d1c1.

[claude]

Claude finished @mchenani's task —— View job

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PR Review - December 15, 2025, 14:09 UTC

Tasks

• Read modified files to understand changes
• Analyze code quality and implementation
• Check for potential bugs or issues
• Review test coverage
• Provide feedback

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Summary

This PR moves the fallback text for LeaveRequest content type from being hardcoded as None to a defined constant and exposes it via an FFI function. The implementation is clean and follows the existing patterns in the codebase.

Findings

✅ Code Quality

• Implementation is idiomatic and consistent with codebase conventions
• The new FALLBACK_TEXT constant is properly documented
• The FFI function follows the same pattern as other decode functions

⚠️ Test Coverage

• Missing test: The new leave_request_fallback() FFI function at bindings_ffi/src/mls.rs:3144 has no test coverage
• Existing test at bindings_ffi/src/mls/tests/content_types.rs:1269 (test_leave_request_encode_decode) doesn't verify the fallback text in the encoded content
• Consider adding:

  #[tokio::test]
  async fn test_leave_request_fallback() {
      let fallback = leave_request_fallback();
      assert_eq!(fallback, "A member has requested leaving the group");
      
      // Also verify it matches what's in encoded content
      let leave_request = FfiLeaveRequest { authenticated_note: None };
      let encoded_bytes = encode_leave_request(leave_request).unwrap();
      let encoded_content = EncodedContent::decode(encoded_bytes.as_slice()).unwrap();
      assert_eq!(encoded_content.fallback, Some(fallback));
  }

ℹ️ Minor Observations

• The impl LeaveRequestCodec block at xmtp_content_types/src/leave_request.rs:21 is separate from the main impl block (line 15). While valid, this could be consolidated for consistency
• The fallback text is now set during encoding (xmtp_content_types/src/leave_request.rs:44), which is the correct behavior

Recommendation

The implementation is solid, but adding test coverage for the new FFI function would improve confidence in the change. No blocking issues identified.

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[mchenani]

@cameronvoell we had this discussion last week, I made this POC PR to check if this is what we want?
1- since we don't have a field for fallback texts in all codecs, that makes it messy since we need an extra function per codec to extract the text, we can make it cleaner by either having a single function to return the associated text for each content type.
2- we already have the fallback texts hardcoded in the sdks, so I would like to extract those if we need to do that as a practice.
3- Since we're having this conversation about the fallback texts, I would like to make it easier for integrators to give them an option to make it easier to translate this texts, so instead of giving them a hardcoded text, I would like to introduce a struct to hold not only the predefined english fallback text, but also gives them a code or a key to help them make a translation based on those keys, for example:
ContentTypeFallbackText{
Key: "LeaveRequest-PendingRemove",
Lan: "En",
Value: "use has requested leaving the group"
}
And so on.
wdyt?