Fix Linux tree template API injection in build-dotnet

[Copilot]

build-dotnet was failing because tree table code generation dropped the tree-specific API on Linux. The tree template used platform-dependent newline detection when inserting generated members, so the insertion marker was missed for CRLF output on a LF host.

• Root cause

  • TreeTableTemplate searched for the insertion point using Environment.NewLine, which is \n on Linux.
  • The base generated table text still contained \r\n, so the tree API block was never inserted.

• Generator change

  • Update TreeTableTemplate.tt to use deterministic marker matching:
    • try #endregion\r\n} first
    • fall back to #endregion\n}
  • This aligns the tree template with the existing graph template behavior.

• Generated output sync

  • Update the checked-in TreeTableTemplate.cs to match the T4 source change so repository builds use the corrected generated implementation immediately.

var idx = text.IndexOf("    #endregion\r\n}", StringComparison.Ordinal);
if (idx < 0)
{
    idx = text.IndexOf("    #endregion\n}", StringComparison.Ordinal);
}