fix: reject blanks for required path params

[sapihav]

fair disclaimer: fix vibed with Astra, but reviewed and verified manually

Problem

ElevenLabs CLI v1.1.0 accepts an empty value for the required voice_id path parameter. It then sends the request to /v1/text-to-speech/, which returns a misleading Not Found instead of a local validation error.

Observed on macOS, Apple Silicon.

Reproduction

No credentials or API request are needed:

elevenlabs text-to-speech convert \
  --dry-run \
  --voice-id '' \
  --model-id eleven_flash_v2_5 \
  --text "Hello"

Before this change, the dry run succeeds and includes:

{
  "method": "POST",
  "url": "https://api.elevenlabs.io/v1/text-to-speech/"
}

Without --dry-run, the API returns:

error[api]: Not Found

A dry run does not contact the API or create an audio file; the bug is that it accepts the invalid path value.

After this change, the same command exits with code 3 and reports:

Required path parameter 'voice_id' is empty. Provide it via --voice-id or --params

Fix and verification

Add is_blank_path_value into validation loops

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Before

After