Upgrade environment schema to support additional scenarios

[toniher]

The schema now accepts two additional scenarios for conda environment definitions:

1. Bare pip dependency (5 lines added)

   • Allows specifying pip as a standalone string dependency
   • Enables pip-based installs in the environment

2. Flexible pip package specifications (refactored pip packages validation)

   • Previously: only accepted version-pinned format (package==version)
   • Now accepts either:
     • Version-pinned packages: package==version (maintains reproducibility)
     • Direct URLs: http:// or https:// links to package artifacts (e.g., wheel files)

Motivation: GitHub issue #11652 — this expands the schema to handle more real-world dependency scenarios while preserving reproducibility where pinning is specified.

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Message created by Claude Haiku 4.5

[toniher]

Hey @mashehu , any feedback or further idea?