feat: add option to force re-running activation scripts between depe…

[Premkumar-2004]

Description

Adds reactivate option to tasks. When set to true, it forces re-running activation scripts before the task executes.
activation:
scripts = ["ros2_ws/install/setup.sh"]

tasks:
build-ros2 = { cwd = "ros2_ws", cmd = "colcon build" }
pyright-ros2 = { cmd = "basedpyright", depends-on = ["build-ros2"], reactivate = true }

Fixes #5433

How Has This Been Tested?

All existing tests pass (cargo test --package pixi_manifest --package pixi_task --package pixi_cli)

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

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[EzraBrooks]

This looks like a great way to do this, assuming (@Hofer-Julian @ruben-arts) we'd prefer this be opt-in vs opt-out.

For ROS users, it being opt-in will probably be a common rake to step on - most people will want reactivation to happen.

However, for other users who might not need this by default, if their environment activation is non-idempotent it could cause weird issues to have it be opt-out. Thoughts?

[Premkumar-2004]

Went with opt-in to avoid breaking existing workflows. Happy to change the default if needed.