Fix generated string-sequence marshalling for messages with multiple string[]/wstring[] fields

[Copilot]

Publishing could fail for messages containing multiple string-array fields (including nested messages), with intermittent NullReferenceException surfacing from publisher buffer creation. The failure pattern matched native message state corruption during generated write-marshalling of dynamic string sequences.

• Root cause in generated WriteTo(ref Priv ...) marshalling

  • For variable-length string[] / wstring[] fields, generated code reassigned native sequence structs directly.
  • Existing native sequence storage was not explicitly finalized before reallocation, which can leave invalid native state when multiple string-sequence fields are present.

• Generator change

  • Updated MessageClassBuilder emission logic for dynamic string arrays to finalize prior native sequence state before allocating replacement storage:
    • CStringSequence: emit priv.<field>.Dispose(); before new CStringSequence(...)
    • U16StringSequence: emit priv.<field>.Dispose(); before new U16StringSequence(...)
  • Change is scoped only to variable-length string/wstring array branches; other field marshalling paths are unchanged.

• Effect

  • Generated message writers now perform safe replace semantics for native string sequences, avoiding invalid state when multiple string-array members are populated in the same message.

// now emitted for dynamic string[] fields
priv.Works.Dispose();
priv.Works = new global::Rosidl.Runtime.Interop.CStringSequence(this.Works.Length);
var Works_span = priv.Works.AsSpan();
for (int __i = 0; __i < this.Works.Length; __i++)
{
    Works_span[__i].CopyFrom(this.Works[__i], textEncoding);
}

[Copilot]

Pull request overview

Updates the C# message code generator to prevent native string-sequence state corruption when marshalling messages that contain multiple variable-length string[] / wstring[] fields.

Changes:

• For dynamic (variable-length) string[] fields, generated WriteTo(ref Priv ...) now finalizes the existing CStringSequence via Dispose() before allocating a replacement sequence.
• For dynamic (variable-length) wstring[] fields, generated WriteTo(ref Priv ...) now finalizes the existing U16StringSequence via Dispose() before allocating a replacement sequence.

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