fix: prevent package updates when appending channels

[benmoss]

Description

When running pixi workspace channel add, packages were being unnecessarily re-solved because any channel change was treated as requiring a full re-solve.

Due to channel priority semantics, appending channels (lower priority) doesn't affect existing package selections - packages from higher priority channels remain valid. Only prepending, removing, or reordering channels should trigger a re-solve.

This change distinguishes between:

• ChannelsExtended: channels were appended (packages stay valid)
• ChannelsMismatch: channels were changed in other ways (need re-solve)

Fixes #5077

How Has This Been Tested?

Creating a pixi project to reproduce the scenario:

pixi init
pixi add python==3.14.0
# manually edit the python constraint in pixi.toml to "*"

pixi workspace channel add https://prefix.dev/nvidia

On a build from this change the pixi.lock is only appending the channel:

diff --git i/pixi.lock w/pixi.lock
index 9252448..8c294ec 100644
--- i/pixi.lock
+++ w/pixi.lock
@@ -3,6 +3,7 @@ environments:
   default:
     channels:
     - url: https://conda.anaconda.org/conda-forge/
+    - url: https://prefix.dev/nvidia/
     options:
       pypi-prerelease-mode: if-necessary-or-explicit
     packages:

as opposed to regular pixi which upgrades python as described in #5077

@@ -3,6 +3,7 @@ environments:
   default:
     channels:
     - url: https://conda.anaconda.org/conda-forge/
+    - url: https://prefix.dev/nvidia/
     options:
       pypi-prerelease-mode: if-necessary-or-explicit
     packages:
@@ -18,7 +19,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/libzlib-1.3.1-h8359307_2.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/ncurses-6.5-h5e97a16_3.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/openssl-3.6.0-h5503f6c_0.conda
-      - conda: https://conda.anaconda.org/conda-forge/osx-arm64/python-3.14.0-h40d2674_103_cp314.conda
+      - conda: https://conda.anaconda.org/conda-forge/osx-arm64/python-3.14.2-h40d2674_100_cp314.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/python_abi-3.14-8_cp314.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/readline-8.3-h46df422_0.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/tk-8.6.13-h892fb3f_3.conda

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added sufficient tests to cover my changes.

[ruben-arts]

Thank you, this looks good. I would like to make the test faster as this could be slowing down our pipelines.

[benmoss]

Just because I did initially struggle with false positives on this test, I made sure that this test fails on main:

E               AssertionError: '0.1.0'
E                not found in stdout:
E                Name     Version  Build           Size  Kind   Source
E               package  0.2.0    h60d57d3_0  1.23 KiB  conda  file:///Users/mossity/workspace/pixi/tests/data/channels/channels/multiple_versions_channel_1

[ruben-arts]

Thank you, it works for me!