feat(tasks): support multi-line tasks

[baszalmstra]

Description

deno_task_shell collapses bare newlines to whitespace, so a multi-line TOML task like

[tasks]
test = """
echo hello
echo hello2
"""

was executed as echo hello echo hello2, printing hello echo hello2.

The fix joins non-empty, non-comment lines with && before parsing, so each line runs as a separate command and execution stops on the first failure. Backslash line continuations are merged, and lines already ending in a shell control operator (&&, ||, |, ;, &) aren't double-separated.

Fixes #5937

How Has This Been Tested?

• Added unit tests test_as_script_multiline and test_as_script_multiline_preserves_existing_operator in crates/pixi_task/src/executable_task.rs.
• Ran the repro from the issue locally; output is now:

  Pixi task (test): echo hello
  echo hello2
  hello
  hello2
  

• Verified fail-fast: a middle false line aborts the task with exit code 1 and the subsequent line is not run.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude (Claude Code)

Checklist:

• I have performed a self-review of my own code
• I have added sufficient tests to cover my changes.

---

Generated by Claude Code

[bollwyvl]

While the current behavior was a little surprising at first, I think most folk have adapted and likey rely on it,

One place where the "newlines become whitespace" is useful is to shell out to some workspace-managed executor, especially with args, which might have its own whitespace/quote/escape rules which probably wouldn't be caught by the home-rolled parser.

cmd = """python -c "import stuff; [
  stuff.do(),
  stuff.do_another('''{{ some_arg}}''')
]" """

Anyhow, as-is, this behavior will be a fairly brutal breaking change. Perhaps it could be opt-in, initially, or at least be opted out of at the workspace or task level?

[baszalmstra]

If I implemented this correctly, the example you gave should not change, except that the newlines become whitespaces. I make sure to break up newlines only when not inside a ' or ".

[bollwyvl]

Sure, I see the caveat about the python case right in the comment; my concern is just that cmd already goes through n parsers (minijinja, maybe some extra-kooky env stuff?, finally d_t_s). Pretty much any change to the effective grammar is going to wreck somebody's day, even if it isn't necessarily mine (which will likely already have been wrecked by n-pass parsers in recipe.yaml (better than most) and .gitlab-ci.yml (kind of a nightmare)).

[bollwyvl]

and lo, this did indeed ruin my day (and at least itk's)