reproduction: pruneMessages leaves reasoning files when reasoning is set

[ai-sdk-factory]

Bug

pruneMessages leaves reasoning files when reasoning is set to all

Reproduction

• examples/ai-functions/src/reproduction/prune-messages-reasoning-files.ts reproduces the bug on main at 9528712c36.
• reasoning: 'all' removes reasoning text but retains reasoning-file; regular file and text parts remain intact.
• reasoning: 'before-last-message' retains reasoning-file in older messages while correctly preserving all reasoning in the last message.
• The failure occurs with synthetic messages and responseMessages produced by both generateText and streamText; the retained reasoning file is forwarded to a subsequent model call.
• A message containing only reasoning text and a reasoning file incorrectly remains after pruning instead of being removed as empty.
• packages/ai/src/generate-text/prune-messages.ts filters only part.type !== 'reasoning', omitting the supported reasoning-file content type.

Related Issues

Reproduces #21102