FIX: Preserve text errors for initializer 403 responses

[biefan]

Summary

• Reuse response-detail extraction for initializer registration 403 responses.
• Fall back to plain text response bodies when a server or proxy returns non-JSON errors.
• Add regression coverage for plain-text 403 responses during initializer registration.

Tests

• uv run -m pytest tests/unit/cli/test_api_client.py tests/unit/cli -q
• uv run -m ty check pyrit/cli/api_client.py tests/unit/cli/test_api_client.py
• uv run pre-commit run --files pyrit/cli/api_client.py tests/unit/cli/test_api_client.py