Fix prompt template names with slashes

[brunojmr08-max]

Closes #254.

Summary

• double-encode slashes in prompt template path segments so they survive an edge/router decode
• keep other special characters encoded once
• cover sync and async template fetches

Tests

$env:PYTHONPATH=(Get-Location).Path
& '..\prompt-layer-library-master\.venv\Scripts\python.exe' -m pytest tests\test_get_prompt_template.py -q
& '..\prompt-layer-library-master\.venv\Scripts\python.exe' -m pytest tests\test_get_prompt_template.py tests\test_agents\test_arun_workflow_request.py -q