fix(runtime): validate and create storage path on init

[Emart29]

Description

Fixes the bug where Runtime accepts a nonexistent storage path without validation, leading to silent failures when trying to save data later.

Fixes #1870

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

• Added path validation in Runtime.__init__()
• Logs a warning when the storage directory doesn't exist
• Auto-creates the directory with parents=True to prevent downstream errors
• Implements Option 3 from the issue discussion (explicit, safe, informative)

Before

runtime = Runtime(Path('nonexistent'))  # Silently accepts
# ... later fails when trying to save

After

runtime = Runtime(Path('nonexistent'))
# WARNING: Storage path does not exist, creating: nonexistent
# Directory is created, saves work correctly

Testing

• Manual testing with nonexistent paths
• Manual testing with existing paths
• Lint passes (ruff check)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• My changes generate no new warnings

[Emart29]

@austin931114 @Hundao

Implemented the fix as discussed in #1870. Ready for review.