test(core): normalize ambient SHELL for Windows shell tests

[dpviivqb]

Issue for this PR

Fixes #54104

Type of change

• Bug fix

What does this PR do?

Windows shell tests hard-code PowerShell syntax, but shell selection prefers process.env.SHELL, which Git Bash sets to /usr/bin/bash. Running the core suite from Git Bash therefore selected bash and every platform-branched command failed with bash syntax errors, while CI stayed green because its bash steps don't propagate SHELL. This deletes SHELL in test/preload.ts on win32 so tests exercise the platform default shell (PowerShell), the same one Windows CI resolves.

Discovered while running the full core suite on Windows from Git Bash; full reproduction and CI-side evidence are in #54104.

How did you verify your code works?

On Windows 11, from a Git Bash environment with SHELL=/usr/bin/bash, using the project runner (bun run test in packages/core):

• without the change: test/tool-shell.test.ts exits 1 with /usr/bin/bash: -c: line 1: syntax error near unexpected token failures
• with the change: 108/108 pass in test/tool-shell.test.ts, and test/shell.test.ts (which drives SHELL explicitly per test) still passes
• also verified a single ShellTool test flips red/green by adding/removing SHELL alone
• bun run typecheck in packages/core is clean

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[opencode-agent]

The change looks right to me, but I couldn't run it: it only does something on Windows, and our review sandbox is Linux. From reading the code: ShellSelect checks process.env.SHELL before anything else, so an inherited SHELL=/usr/bin/bash from Git Bash would make the shell tests pick bash while they expect PowerShell. Removing it in the test preload runs before any shell is chosen, and test/shell.test.ts sets or removes SHELL in each test itself, so its Windows and Git Bash tests aren't affected. No blocking concerns. A maintainer on Windows can confirm it by running SHELL=/usr/bin/bash bun run test test/tool-shell.test.ts in packages/core from Git Bash.