fix: return all tools from paginated MCP tool lists (#19246)

## Background

MCP servers with paginated tool lists exposed only first-page tools
through client.tools(), silently making later tools unavailable.

## Root Cause

DefaultMCPClient.tools() called listTools() once and ignored nextCursor,
despite the response schema and listTools() API supporting cursors; the
reproduction confirmed only one tools/list request was sent.

## Summary

Updated tools() to follow each nextCursor and combine all returned tool
definitions before creating the tool set. Added a patch changeset for
@ai-sdk/mcp.

## Testing

Added a regression test verifying that tools() requests the continuation
cursor and returns tools from both pages.

## End-to-end Validation

- `git show
HEAD:examples/ai-functions/src/reproduction/issue-19240-mcp-tools-pagination.ts
| pnpm -C examples/ai-functions exec tsx -` returned all three tools and
recorded requests with cursors `null` and `page-2`.

## Related Issues

Fixes #19240

Closes #19242

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: habibthadev <109099706+habibthadev@users.noreply.github.com>