fix: discover only text models for chat

[voodoohop]

• Load the text catalog for the chat selector instead of the all-modality /v1/models catalog.
• Pollinations model renaming gives image models publisher-prefixed IDs, so the old exact-name exclusion no longer removes them. They would be sent to chat completions, where they are unsupported.
• The existing array parser already accepts the text catalog. Verified the live response: 273 string-named entries, all supporting chat completions. No generation requests made.
• Flutter/Dart tests were not run: neither SDK is installed here.