fix(shell-hook): make prompt modification idempotent to prevent duplication

[mango766]

Summary

Fixes #5599

Running eval "$(pixi shell-hook ...)" multiple times for the same environment currently causes:

• Bash/Zsh: PS1 prefix keeps getting prepended → (env) (env) (env) $
• PowerShell: Recursive wrapping of the prompt function → stack overflow crash
• Nu Shell: Indefinite nesting of PROMPT_COMMAND

Root cause

The prompt modification functions in crates/pixi_core/src/prompt.rs (posix_prompt, powershell_prompt, nu_prompt) unconditionally modify the shell prompt on every invocation, with no check for whether the prompt was already modified.

Notably, the fish_prompt function already handles this correctly by checking if not functions -q __fish_prompt_orig before wrapping.

Fix

Add an idempotency guard using a __PIXI_PROMPT_MODIFIED environment variable. Each prompt function now:

1. Checks if __PIXI_PROMPT_MODIFIED is already set to the expected value for the current environment
2. Only modifies the prompt if the guard is not set (i.e., first activation)
3. Sets the guard after modifying the prompt

This also correctly handles switching between environments — if you activate env A then env B, the guard value changes so the prompt gets updated.

Shells affected

Shell	Before	After
Bash/Zsh	PS1 duplicated on each eval	Idempotent ✅
PowerShell	Stack overflow on repeated eval	Idempotent ✅
Nu Shell	Nested prompt wrapping	Idempotent ✅
Fish	Already idempotent	Unchanged ✅
Cmd.exe	Already idempotent (@PROMPT replaces)	Unchanged ✅
Xonsh	No-op	Unchanged ✅

Test plan

• Run eval "$(pixi shell-hook -m <project>)" twice in bash — PS1 should show (env) only once
• Run (pixi shell-hook) | Out-String | Invoke-Expression twice in PowerShell — no stack overflow
• Switch environments: activate env A then env B — prompt should update to B
• Existing test_shell_hook_unix / test_shell_hook_windows tests should still pass

[baszalmstra]

• But this still results in the same behavior when you activate two different environments.
• Does this work correctly if you do this for two different projects with the same environment name?