refactor: migrate featherless provider to AI SDK (#11265)

* refactor: migrate featherless provider to AI SDK

* fix: merge consecutive same-role messages in featherless R1 path

convertToAiSdkMessages does not merge consecutive same-role messages
like convertToR1Format did. When the system prompt is prepended as a
user message and the conversation already starts with a user message,
DeepSeek R1 can reject the request.

Add mergeConsecutiveSameRoleMessages helper that collapses adjacent
Anthropic messages sharing the same role before AI SDK conversion.
Includes a test that verifies no two successive messages share a role.

Author: daniel-lxs