fix: human tool approval requests omit policy reasons (#19658)

## Background

OPA documentation promises that `requires-approval` reasons reach human
approvers, but published generation and UI approval requests omitted
them.

## Root Cause

The OPA normalizer discarded `requires-approval` reasons, while
user-approval status and request types could not carry them; the
original reproduction confirmed both `generateText` and `streamText`
lost the reason.

## Summary

- Preserved optional reasons through OPA normalization, approval status
and request types, `generateText` and `streamText` outputs, response
messages, UI serialization, processing, validation, and model-message
conversion.
- Kept the UI request reason as `approval.requestReason`, separate from
the approver's response `reason`, so both survive approval state
transitions and persisted-message round trips.
- Documented the core result, UI message, stream protocol, and
model-message conversion fields.
- Added patch changesets for `ai`, `@ai-sdk/policy-opa`, and
`@ai-sdk/provider-utils`.

## Compatibility

Existing approval inputs and stored messages remain valid. Requests
without a reason keep their previous runtime shape. The UI stream adds
only an optional `reason` field, while UI messages add the optional
`approval.requestReason` field and retain the existing response
`approval.reason` semantics.

## Testing

- Focused AI UI tests: 270 passed in Node and 270 passed in Edge.
- Focused policy-OPA tests: 22 passed in Node and 22 passed in Edge.
- Affected package type checks and the full workspace type check passed.
- Formatting and lint checks passed with no warnings or errors.
- Documentation content sync, MDX generation, and docs TypeScript
checking passed.

Regression coverage verifies OPA normalization and integration,
`generateText`, `streamText`, response-message conversion, UI chunk
conversion and processing, request/response reason lifecycle
preservation, stored-message validation, public types, and model-message
conversion.

## End-to-end Validation

- `replay_original_reproduction`: both `generateText` and `streamText`
human approval requests contained the policy reason; classified
`no-longer-reproduces`.

## Related Issues

Fixes #19654

Closes #19655

---------

Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: ladanjohari <13539962+ladanjohari@users.noreply.github.com>
Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>