Fix format specifiers and arguments in com_dotnet (GH-15398)

This is mostly about minor glitches (signedness or length confusion),
but also fixes two occasions where `zend_string`s still

Fix format specifiers and arguments in com_dotnet (GH-15398)

This is mostly about minor glitches (signedness or length confusion),
but also fixes two occasions where `zend_string`s still have been
regarded as `char *`.

We also add a regression test case for failing property name lookup,
since that is the most relevant issue we're fixing here.

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