text: Title: Refine handling and display of the `X-Schemathesis-TestCaseId` header
  Body: The `X-Schemathesis-TestCaseId` header is currently displayed as part of the reproduction command. This inclusion is misleading because the ID in the generated command was not actually sent during testing. As a result, it can't be used for searching through logs based on this ID.
  
  - Remove `X-Schemathesis-TestCaseId` from the CLI output for the reproduction command.
  - Retain `X-Schemathesis-TestCaseId` only in VCR cassettes for traceability.
  - Document how this header behaves and why it could be useful
  - Modify the ID format to use an alphabet with more than just hexadecimal characters for better readability and shorter length.
  