fix: OpenAI Responses streams silently dropping schema-invalid known events and reporting stop (#19495)

## Background

Schema-invalid OpenAI Responses events could silently discard completed
tool calls and make an invalid stream appear to have completed
successfully.

## Root Cause

The permissive unknown-event fallback converted malformed known events
into ignored chunks, including malformed output-item envelopes and an
unmodeled function-call-arguments completion event, while
response.completed could overwrite the resulting error finish reason
with stop.

## Summary

Known malformed Responses events now emit stream errors,
response.function_call_arguments.done is validated, malformed
output-item envelopes cannot enter the unknown fallback, error finish
reasons are preserved, genuinely future event and item types remain
ignored, and the existing patch changeset covers the behavior.

## Testing

Regression coverage verifies malformed known events, output-item
envelope variants, valid and malformed function-call-arguments
completion events, error finish preservation, and forward compatibility
for unknown event and item types. OpenAI package tests, AI package Node
and Edge tests, the full workspace suite, lint, formatting, and full
type checking pass.

## End-to-end Validation

- `pnpm -C examples/ai-functions exec tsx -e '<Responses stream
replay>'` produced one tool call with `tool-calls` for the valid control
and four errors with an `error` finish reason for the schema-invalid
stream.

## Related Issues

Fixes #19493

Closes #19494

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: Sparky579 <48359461+Sparky579@users.noreply.github.com>
Co-authored-by: ai-sdk-factory[bot] <305873210+ai-sdk-factory[bot]@users.noreply.github.com>
Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>