fix(core): clear ARGV0 before spawning terminals

[zonemeen]

Issue for this PR

Closes #40839

Type of change

• Bug fix

What does this PR do?

AppImage sets ARGV0 to its executable path. When inherited by zsh, it overrides child commands' argv[0], causing Rustup proxies such as cargo and rustc to fail with unknown proxy name in the integrated terminal.

• Remove ARGV0 from the PTY environment before spawning the shell.
• Add a regression test that checks ARGV0 is removed while preserving terminal variables and the parent process environment.

How did you verify your code works?

• Confirmed the new regression test fails before the fix and passes after it.
• Built and ran before/after Linux ARM64 AppImages under Xvfb using --appimage-extract-and-run. Through the bundled backend's PTY API, verified that cargo --version, rustc --version, and an offline Rust program fail before the fix and succeed after it.
• Passed 25 PTY/Shell tests on both macOS and Linux. One existing Linux lifecycle test initially timed out and passed on rerun.
• Passed core type checks, formatting checks, and git diff --check.

Screenshots / recordings

Rendered from captured AppImage PTY output; command labels added and long valid-proxy lists omitted.

Before	After
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Checklist

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