Fix slash handling in PromptLayer API path segments

[Jorel97]

Closes #254

Summary

• add a shared path-segment encoder that double-encodes / so prompt and workflow names survive one edge/router decode pass
• use it for sync and async prompt template fetches
• use it when resolving workflow names
• add regression coverage for slash-containing prompt and workflow names

Verification

• python -m pytest tests\test_get_prompt_template.py tests\test_agents\test_arun_workflow_request.py -q
• python -m ruff check promptlayer\utils.py tests\test_get_prompt_template.py tests\test_agents\test_arun_workflow_request.py
• python -m ruff format --check promptlayer\utils.py tests\test_get_prompt_template.py tests\test_agents\test_arun_workflow_request.py