Avoid std::back_insert_iterator of fmt::memory_buffer in fmt::format

[oleksandr-pavlyk]

closes #396

fmt 12.2 changed the implementation path used by fmt::format_to with std::back_insert_iterator. Under nvcc, that path rejects std::back_insert_iterator<fmt::memory_buffer> because fmt’s constexpr helper takes a non-literal iterator type. Replacing std::back_inserter(buffer) with fmt::appender(buffer) for fmt::memory_buffer outputs avoids that path and is the fmt-native way to append to memory_buffer.

[oleksandr-pavlyk]

@coderabbitai full review

[coderabbitai]

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Updated several output-generation paths to use a more efficient formatting approach.
  • Kept CSV and Markdown output content and structure unchanged.
• Tests
  • Aligned test output-building code with the updated formatting approach.
  • No changes to expected results or test behavior.

Walkthrough

All fmt::format_to calls writing into fmt::memory_buffer instances replace std::back_inserter(buffer) with fmt::appender(buffer) across library source (csv_printer, markdown_printer, markdown_table, state) and test files. markdown_table::to_string() additionally replaces its std::vector<char> intermediate buffer with fmt::memory_buffer.

fmt::appender migration

Layer / File(s)	Summary
Library formatters nvbench/csv_printer.cu, nvbench/markdown_printer.cu, nvbench/internal/markdown_table.cuh, nvbench/state.cxx	All production fmt::format_to calls into fmt::memory_buffer switch to fmt::appender; markdown_table::to_string() also replaces its std::vector<char> + std::back_inserter buffer with fmt::memory_buffer + fmt::appender + fmt::to_string.
Test helpers and test functions testing/axes_metadata.cu, testing/benchmark.cu, testing/create.cu, testing/option_parser.cu, testing/runner.cu, testing/state_generator.cu	All test-side fmt::format_to calls into fmt::memory_buffer switch from std::back_inserter to fmt::appender; no test logic, expected strings, or control flow changed.

Assessment against linked issues

Objective	Addressed	Explanation
Replace std::back_inserter(buffer) with fmt::appender(buffer) when writing into fmt::memory_buffer to fix compilation with fmt 12.2 [#396]	✅
For code using std::vector<char> + std::back_inserter solely to build a string, switch to fmt::memory_buffer + fmt::appender [#396]	✅

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[oleksandr-pavlyk]

The CI run using fmt 11.2 is green. I also opened #398 where version of fmt was bumped to 12.2 and also includes this change. That build is green as well.