[repo-assist] Add regression test confirming square brackets are escaped in nupkg entries (#3906)

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Relates to #3906 ("paket pack doesn't escape square brackets").

Investigation

I traced the packing pipeline in NupkgWriter.fs (ensureValidName, used by addEntryFromFile) and tested it directly against a file named file[0].txt. In the current codebase, this path already produces a correctly escaped zip entry name (file%5B0%5D.txt), not the unescaped file[0].txt reported in the issue. I could not find any git history since the issue was filed indicating a targeted fix, so this may have been resolved indirectly (e.g. via the Uri-based round-trip in ensureValidName), or the original repro used a different, older Paket version.

Since I can't rule out a repro path I haven't hit, I'm not claiming the issue is fully resolved — but I've added a regression test to lock in and document the current (correct) behavior, so any regression here would be caught immediately.

Change

• New test tests/Paket.Tests/Packaging/NupkgWriterSpecs.fs: packs a folder containing file[0].txt via NupkgWriter.Write and asserts the resulting nupkg's zip entry is lib/net461/file%5B0%5D.txt (escaped), not the raw bracketed name.
• Registered the new file in Paket.Tests.fsproj.

No production code changes — this is a pure test addition intended to give maintainers confidence and a safety net while they decide on the final disposition of #3906.

Test Status

• dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~NupkgWriter": Passed on net10.0 (1/1).
• The net461 target aborted in this sandbox only because mono is not installed here (infrastructure limitation, unrelated to the change) — worth re-running in CI where mono is available.

I'll also leave a comment on #3906 summarizing this finding so maintainers can confirm whether the issue is fully closed or if there's another repro path to track down.

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