profiling: --collapsed export fails with UnicodeEncodeError on non-ASCII frame names

Bug report

Bug description:

CollapsedStackCollector.export (Lib/profiling/sampling/stack_collector.py) opens its output with open(filename, "w") — no encoding=. It is the only exporter in that module missing it; FlamegraphCollector.export, the JSONL exporter and the heatmap exporter all pass encoding="utf-8". Because the collapsed format writes frame names verbatim (the JSON exporters escape non-ASCII via ensure_ascii), --collapsed export raises UnicodeEncodeError when a sampled frame's function or file name contains non-ASCII characters under a non-UTF-8 locale, or surrogate-escaped characters (from an undecodable path) on any locale:

UnicodeEncodeError: 'ascii' codec can't encode characters ...
UnicodeEncodeError: 'utf-8' codec can't encode character '\udc80' ... surrogates not allowed

Non-English identifiers are common and non-UTF-8 locales are supported, so profiling such a program with --collapsed produces no output and an uncaught error.

CPython versions tested on:

CPython main

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-156811
• gh-156814

Start with Lib/profiling/sampling/stack_collector.py and compare CollapsedStackCollector.export with the other exporters that specify UTF-8 output. Check linked PRs gh-156811 and gh-156814 first; done means collapsed export succeeds for non-ASCII and surrogate-escaped frame names without UnicodeEncodeError.