The chat-completions refactor removed the legacy Perplexity "text"
extraction but left two tests asserting the old behavior, breaking
make check on every CI flavor:
- test_ai_parse_response_perplexity_text expected the legacy /v1/agent
nested format to parse; it now yields NULL — renamed to
test_ai_parse_response_legacy_text_format_unsupported.
- test_ai_parse_response_text_preferred_over_content expected "text"
to win; "content" is authoritative now — renamed to
test_ai_parse_response_content_preferred_over_text.
Add test_ai_set_provider_setting_reserved_key covering the new
reserved-key rejection.