Remove preview extension bundles from AFD v2

[andystaples]

Summary

• remove preview extension bundle guidance from the AFD v2 migration guide and samples README
• configure all sample apps to use the public Azure Functions extension bundle
• configure the AFD v2 E2E apps to enforce support through CI

The public Azure Functions extension bundle now includes an AFD v2-compatible version.

[Copilot]

🟡 Changes recommended

The PR makes a user-facing behavior/guidance change but does not add the required azure-functions-durable/CHANGELOG.md entry under ## Unreleased.

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Pull request overview

Updates the Azure Functions Durable (AFD) v2 documentation and sample/E2E apps to stop relying on the preview Azure Functions extension bundle now that the public bundle includes an AFD v2-compatible version.

Changes:

• Switched sample and E2E host.json files from Microsoft.Azure.Functions.ExtensionBundle.Preview to Microsoft.Azure.Functions.ExtensionBundle.
• Removed preview-bundle guidance from the AFD v2 migration guide and samples README.

File summaries

File	Description
tests/azure-functions-durable/e2e/apps/v1_style/host.json	Updates E2E app to use the public extension bundle ID.
tests/azure-functions-durable/e2e/apps/tracing/host.json	Updates tracing E2E app to use the public extension bundle ID.
tests/azure-functions-durable/e2e/apps/dtask_style/host.json	Updates durabletask-style E2E app to use the public extension bundle ID.
azure-functions-durable/samples/README.md	Removes preview-bundle requirement text from sample run guidance.
azure-functions-durable/samples/human-interaction/host.json	Updates sample to use the public extension bundle ID.
azure-functions-durable/samples/function-chaining/host.json	Updates sample to use the public extension bundle ID.
azure-functions-durable/samples/fan-out-fan-in/host.json	Updates sample to use the public extension bundle ID.
azure-functions-durable/samples/durable-entities/host.json	Updates sample to use the public extension bundle ID.
azure-functions-durable/MIGRATION_GUIDE.md	Removes the migration step instructing use of the preview extension bundle.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Lite

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[Bernd Verst (berndverst)]

The stable bundle switch is correct, but the migration path still needs to tell existing 1.x apps to move off older bundle majors before adopting the gRPC-based 2.x provider.

[Bernd Verst (berndverst)]

Please retain an extension-bundle upgrade step here, replacing the preview ID rather than deleting the requirement. This guide targets existing 1.x apps, which may still be pinned to bundle 3.x ([3.3.0, 4.0.0)); that bundle carries DurableTask 2.13.7, while this provider marks every Durable binding durable_requires_grpc=true and needs the DurableTask 3.x host extension now available through the stable 4.x bundle. As written, users can follow every migration step without updating an incompatible host.json. Showing Microsoft.Azure.Functions.ExtensionBundle with [4.*, 5.0.0) here would preserve the required migration guidance.