Function to_session_path 

pub fn to_session_path(input: &str) -> String

Canonical leading-slash session path from any accepted spelling: collapses repeated slashes, strips the /workspace alias, ensures a single leading slash, and trims a trailing slash.

This is the single normalizer for every session-path surface — the agent (via MountFs/the VFS backends) and the control-plane HTTP FS API both route through it, so a path resolves to the same key regardless of entry point.

Examples: /workspace → /, /workspace/a.txt → /a.txt, /workspacefoo → /workspacefoo, a.txt → /a.txt, /sub/dir/ → /sub/dir, /a//b/ → /a/b.