feat: add resolve-symlinks option to control symlink behavior in manifest paths

[claydugo]

Description

Adds a resolve-symlinks option to the [workspace] section that controls
how pixi handles symlinked manifest files.

By default (true), pixi fully resolves symlinks to determine the workspace root from the real file location.
This restores the original behavior that was changed in #4912 and #5059, fixing the regression reported in #5148.

How Has This Been Tested?

I built the binary locally and tested both workflows by setting the new resolve-symlinks flag.

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 Opus 4.5

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.