test(preview): feed the stubbed responses on stdin, not through a path

[L4XB]

Refs #329.

The defect

The test wrote its canned curl responses to a temp file and handed Bash the path. On Windows that path is C:\…\responses, and Git Bash reads the backslashes in

exec 3< "C:\…\responses"

as escapes — the redirect fails with No such file or directory and the shell exits before the first loop iteration. Every case then reports status 1 and zero attempt lines, exactly as the issue describes, while Linux CI stays green.

Demonstrable without Windows, since it is the quoting that breaks rather than the filesystem:

$ bash -c 'exec 3< "C:\tmp\resp"'
bash: C:\tmp\resp: No such file or directory

The fix

The responses arrive on stdin (spawnSync's input) and the script duplicates fd 0:

exec 3<&0

There is now no path to survive the trip into Bash, so the failure mode cannot occur on any platform — rather than being translated around with cygpath or a backslash rewrite, which would leave the same shape one quoting rule away from breaking again.

It keeps the property the temp file was there for, and which the comment names: one shared descriptor that advances even inside curl's command substitution. The temp directory, its try/finally cleanup, the PREVIEW_RESPONSES variable and three imports go with it — the diff removes slightly more than it adds.

Verified

Two mutations, each breaking the shared-fd property in a different way:

mutation	result
exec 3< /dev/null instead of stdin	8 of 11 fail
no input handed to bash	8 of 11 fail

The three that survive both are the single-response cases, where the first read is all that matters — which is the right shape: a test that passes on one response says nothing about a descriptor advancing.

$ npx vitest run src/lib
Test Files  4 passed (4)
     Tests  34 passed (34)

$ npx prettier --check src/lib/pr-preview-access.test.ts
All matched files use Prettier code style!

$ npx tsc --noEmit
(clean)

I could not run this on Windows — no Windows machine here. The argument that it fixes #329 is structural (there is no longer a path for Git Bash to mis-quote) plus the quoting demonstration above, not an observed green run on Windows. Worth one confirmation from a Windows contributor before you trust it.

Related: #331 covers the other half of the Windows story (CRLF checkout), sent separately as #332.