fix(ai): fail malformed converse output

[rekram1-node]

Summary

• fail Bedrock Converse streams instead of emitting a successful finish for malformed_model_output and malformed_tool_use
• surface an InvalidProviderOutput error with the raw stop reason and triggering event frame
• preserve model_context_window_exceeded as a normal length finish

Testing

• bun test test/provider/bedrock-converse.test.ts
• bun typecheck
• Prettier and oxlint on the changed files
• git diff --check