[BUG] `cocoindex show` should interpret component keys correctly, and also show "not found" if not found

Example:

~/src/cocoindex/examples/text_embedding $ coco-dev-run cocoindex show -r main '/@process_files/"rfc8259.md"'
Loaded environment variables from: /Users/hjz/src/cocoindex/examples/text_embedding/.env

Stable paths:
  /"@process_files"/"rfc8259.md"
    type:directory version:0 processor:-
    has_memoization:false target_state_count:0

Problems:

1. @process_files is a symbol, but interpreted as a string.
2. It should be "not found" (since it interprets @process_files as string), but it shows an entry any way (actually replacing the input path by anything will end up with an entry like this)

Suggested approach for interpreting StableKey (as key for components) passed to CLI correctly:

• For each type of StableKey that we support, make sure when we convert them to string, their representation doesn't collide.
• Then we can have a method to parse a StablePath out of its string representation.