Fix name collision with "result" in pretty printing logic in generated proto code.

[dconeybe]

When generating the C++ code from the .proto files, it was discovered that if a proto message happens to have a field named "result", "header", or "tail" then the generated C++ code would fail to compile with an error like this:

In file included from firestore.nanopb.cc:22:
pretty_printing.h: In instantiation of ‘std::string firebase::firestore::nanopb::PrintMessageField(...)’:
firestore.nanopb.cc:630:32:   required from here
pretty_printing.h:59:25: error: ‘const class std::__cxx11::basic_string<char>’ has no member named ‘ToString’
   59 |   auto contents = value.ToString(indent_level);
      |                   ~~~~~~^~~~~~~~

The root cause was that the ToString() methods defined local variables named "result", "header", and "tail" which would collide with other local variable names created for the proto messages' fields if they were also coincidentally named "result", "header", or "tail".

The fix in this PR changes those local variable names to "tostring_result", "tostring_header", and "tostring_tail", respectively, to avoid such naming conflicts in the future. Admittedly, if a future proto message happens to have a field named "tostring_result", for example, then this problem will resurface; however, that seems like an unlikely scenario and we'll deal with that if it ever happens.

When reviewing this PR, the only file that was modified by hand was Firestore/Protos/lib/pretty_printing.py; all of the other changes in this PR are the regenerated C++ code from the .proto files.

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Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Overall coverage changed from 88.48% (96450bd) to 88.23% (5bf9be5) by -0.25%.

  15 individual files with coverage change

  Filename	Base (96450bd)	Merge (5bf9be5)	Diff
  common.nanopb.cc	34.52%	34.48%	-0.04%
  document.nanopb.cc	96.52%	96.67%	+0.14%
  firestore.nanopb.cc	39.15%	38.68%	-0.47%
  latlng.nanopb.cc	84.62%	86.67%	+2.05%
  leveldb_mutation_queue.cc	92.42%	93.56%	+1.14%
  maybe_document.nanopb.cc	30.95%	28.89%	-2.06%
  mutation.nanopb.cc	77.42%	75.76%	-1.66%
  ordered_code.cc	94.39%	93.90%	-0.49%
  query.nanopb.cc	87.85%	87.95%	+0.10%
  status.nanopb.cc	75.00%	72.22%	-2.78%
  struct.nanopb.cc	7.59%	7.32%	-0.28%
  target.nanopb.cc	45.95%	45.00%	-0.95%
  task.cc	93.91%	94.78%	+0.87%
  wrappers.nanopb.cc	9.26%	9.40%	+0.14%
  write.nanopb.cc	62.11%	60.23%	-1.88%

Test Logs

• Base (96450bd): https://github.com/firebase/firebase-ios-sdk/actions/runs/2965474157
• Merge (5bf9be5): https://github.com/firebase/firebase-ios-sdk/actions/runs/2983352635

1. https://storage.googleapis.com/firebase-sdk-metric-reports/UKkUgM7JE3.html