feat: support pip style no-deps

[lucifer1004]

Description

Summary:

• Added no-deps support for PyPI dependencies so users can install/lock a package without pulling its transitive requirements.
• For registry packages, the direct requirement is resolved to a concrete version using index metadata and the best compatible wheel (or sdist), but Requires-Dist is ignored. For git/path/url, we lock metadata directly without resolving dependencies.
• This mirrors pip install --no-deps expectations (direct requirement only, no transitive deps) while keeping the lockfile deterministic and avoiding dependency solver influence from no-deps entries.

Context:
Users expect no-deps to skip transitive dependencies but still allow unpinned direct requirements.

Fixes #1417

How Has This Been Tested?

Full test suite passed locally:

• cargo test --workspace

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: Codex (Cursor)

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

If you want this scoped down to only the no-deps behavior (and exclude the stability/doctest fixes), say so and I’ll trim the description and checklist accordingly.