feat(open-responses): support experimental extension codecs (#18900)

## Summary

Adds an experimental extension registry to `@ai-sdk/open-responses` for
namespaced tools, response items, and streaming events.

Each codec can opt into tool, item, and event handling independently
through `experimental_extensions`. Registered provider tools are encoded
into requests, decoded calls and events set execution ownership per
result, and unsupported tools continue to produce warnings. Invalid
provider-tool choices are omitted instead of referencing a tool that was
not sent.

Decoded items retain their original wire representation in one replay
carrier. This keeps opaque fields intact across `generateText` history,
including items that decode only to source parts, without copying the
full payload onto every decoded part.

Models with callback-based extension codecs reject workflow
serialization because the callbacks cannot cross workflow boundaries.

Cross-package coverage lives in the private `tools/integration-tests`
workspace. Neither `ai` nor `@ai-sdk/open-responses` declares a
dependency or devDependency on the other.

## Example

Adds a mocked `generateText` example that registers a document-search
extension and demonstrates tool encoding, provider execution, and
multi-turn replay of source and receipt items.

## Tests

- Open Responses Node and Edge suites: 102 tests each
- Neutral Open Responses/core integration suite: Node and Edge
- Remaining `ai` provider-error integration suite: Node and Edge
- Full repository type check and formatting/lint checks

Fixes #18899

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: me2seeks <96464454+me2seeks@users.noreply.github.com>
Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>