feat(tasks): support aliases in pixi.toml task definitions

[Aditya7880900936]

Summary

Closes #6504

This PR adds support for defining task aliases directly in pixi.toml.

Instead of creating a separate alias task:

[tasks]
docker-start = { cmd = "docker compose up --build -d" }
ds = [{ task = "docker-start" }]

users can now write:

[tasks]
docker-start = { cmd = "docker compose up --build -d", alias = "ds" }

The alias can be used anywhere a task name is accepted:

pixi run ds

while pixi task list continues to display only the original task, avoiding additional alias entries.

Implementation

• Added an optional alias field to executable task definitions.
• Extended TOML parsing and serialization to support the new key.
• Updated task resolution to fall back to matching task aliases when an exact task name is not found.
• Added parser coverage and updated snapshots for the new alias key.

Validation

• cargo check
• cargo test -p pixi_manifest
• cargo test -p pixi_core
• Manual verification:
  • pixi run ds
  • pixi task list

[hunger]

Thank you for your contribution, but I think you are missing a few bits and pieces still:

pixi run -e something ds does not seem to work for me.

You also seem to be missing error handling: What e.g. happens when you reuse a task name as another tasks alias?

it would be wonderful if you could add tests that trigger the above error conditions and then proceed to fix the issues. That will also build up test coverage and help catch more errors.

[hunger]

Sorry, I was on vacation… I'll take another look ASAP.