feat: normalize mid-stream provider error events into public typed errors with provider-owned status and retry metadata (#18671)

## Background

Official providers can report structured failures after streaming
starts, but applications previously received unknown objects or strings
and had to inspect provider payloads or match messages to make retry
decisions. A stable public error contract is needed independently of
automatic stream retry support.

## Summary

Adds the exported marker-based StreamProviderError and normalizes
well-formed mid-stream failures while preserving messages, provider type
or code discriminators, explicit and provider-owned status codes,
conservative retryability, raw payloads, existing same- and cross-realm
Error instances, and identity across callbacks and consumers. Shared
provider-utils metadata transport supplies adapter-owned mappings for
Anthropic, OpenAI, DeepSeek, Moonshot AI, Groq, and xAI, including xAI
response.failed events. Gateway metadata survives serialization,
permanent insufficient_quota failures are non-retryable, numeric HTTP
codes do not replace provider types, and provider specification error
fields remain unknown for compatibility.

## Testing

Runtime and public type coverage includes typed, message-only, nested,
explicit-metadata, retryable, non-retryable, malformed, false-positive,
numeric-code, existing-error, and cross-realm cases. Actual Anthropic,
OpenAI, Gateway, DeepSeek, Moonshot AI, Groq, and xAI adapters are
exercised through streamText in Node and Edge, including raw data,
metadata, and callback/consumer identity. Added permanent
insufficient_quota regressions for OpenAI, DeepSeek, and xAI. Focused
suites and the final full repository test suite passed.

## End-to-end Validation

- The application-managed retry example classified an overloaded event
as retryable with status 529, discarded buffered partial output,
retried, and completed.
- Live OpenAI, DeepSeek, and xAI streams completed successfully in this
iteration.
- Earlier branch validation also completed live Anthropic, Groq, and xAI
streams successfully.

## Documentation

Updates Core error handling, the streamText API reference, AI Gateway
documentation, and the error index; adds the AI_StreamProviderError
reference page; and documents conservative classification, provider
type/code discrimination, partial-output retry considerations, and an
application-managed retry example.

## Related Issues

Fixes #18669

---------

Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@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>