fix(lib): use absolute workspace path with canonicalization

[bug-ops]

Summary

• Fix LSP server initialization failure when no workspace roots are configured
• Add resolve_workspace_roots() helper that uses std::env::current_dir() with canonicalization
• Prevent symlink-based path traversal via canonical path resolution
• Add graceful fallback with warnings on errors

Problem

When no workspace roots were configured, mcpls used PathBuf::from(".") which resulted in file://. being sent to LSP servers. rust-analyzer and other LSP servers don't handle relative URIs, causing "LSP server process terminated unexpectedly" errors.

Solution

Extract workspace root resolution into a dedicated helper function that:

1. Uses std::env::current_dir() to get the absolute path
2. Canonicalizes the path to resolve symlinks (security improvement)
3. Falls back gracefully with warnings if either operation fails

Test plan

• All 108 tests pass (3 new unit tests added)
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo +nightly fmt --check passes
• Manual testing: MCP tools now work correctly with default workspace

Review Notes

• Addressed performance review: extracted helper to reduce double clone pattern
• Addressed security review: added canonicalization for symlink protection
• Addressed testing review: added comprehensive unit tests