Add Prepare Release GitHub Action for automated release kickoff

[YunchuWang]

Summary

What changed?

• Added a new \Prepare Release\ GitHub Actions workflow (.github/workflows/prepare-release.yaml) to automate SDK release preparation, aligned with the existing workflow in durabletask-js.
• Updated doc/release_process.md to document the automated release workflow and restructure the release process documentation.

Why is this change needed?

• The durabletask-js repo already has an automated Prepare Release GitHub Action that handles version bumping, changelog generation, branch/tag creation, and PR link generation. This PR brings the same automation to durabletask-dotnet for consistency and to reduce manual release toil.

What the workflow does

1. Determines next version: Auto-increments from eng/targets/Release.props or accepts explicit version input
2. Generates changelog: Uses git log diff between latest tag and HEAD
3. Updates Release.props: Sets VersionPrefix and VersionSuffix
4. Updates CHANGELOG.md: Inserts new version section after ## Unreleased
5. Creates release branch + tag: release/vX.Y.Z and vX.Y.Z
6. Provides PR creation link in the workflow summary

Issues / work items

• Aligning release process with durabletask-js

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Project checklist

• Release notes are not required for the next release
• Backport is not required
• All required tests have been added/updated (unit tests, E2E tests)
  • N/A - this is a CI workflow, validated by running the workflow itself
• Breaking change? No

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AI-assisted code disclosure (required)

Was an AI tool used? (select one)

• No
• Yes, AI helped write parts of this PR (e.g., GitHub Copilot)
• Yes, an AI agent generated most of this PR

If AI was used:

• Tool(s): GitHub Copilot
• AI-assisted areas/files: .github/workflows/prepare-release.yaml, doc/release_process.md
• What you changed after AI output: Reviewed and finalized

AI verification (required if AI was used):

• I understand the code and can explain it
• I verified referenced APIs/types exist and are correct
• I reviewed edge cases/failure paths (timeouts, retries, cancellation, exceptions)
• I reviewed concurrency/async behavior
• I checked for unintended breaking or behavior changes

[Copilot]

Pull request overview

Adds an automated “Prepare Release” GitHub Actions workflow to kick off SDK releases (version bump + changelog + branch/tag creation) and updates the release process documentation to describe this automation.

Changes:

• Introduces .github/workflows/prepare-release.yaml to automate version calculation, changelog generation, and creation of release/vX.Y.Z + vX.Y.Z.
• Updates doc/release_process.md to document the recommended automated release preparation flow and reorganize publishing steps.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
.github/workflows/prepare-release.yaml	New workflow to bump eng/targets/Release.props, update CHANGELOG.md, and push a release branch/tag.
doc/release_process.md	Documents the new workflow and clarifies the end-to-end release/publishing process.

[YunchuWang]

Addressing review comments:

Comment 1 (silent version parse fallback): This matches the pattern in the durabletask-js sibling workflow intentionally. The version in Release.props is tightly controlled so regex miss is very unlikely. If we want to add validation, it would make sense as a follow-up across both repos for consistency.

Comment 2 (git commit fails on re-runs): Fixed - added git diff --cached --quiet guard so re-runs don't fail when there are no changes to commit.

Comment 3 (force-push risk on tags): The force-push pattern is intentional for re-run support and matches the JS sibling workflow. Since this is a manually-triggered workflow_dispatch only accessible to maintainers, the risk of accidental overwrite is low. Published releases are further protected by the ADO pipeline being a separate manual step.

Comment 4 (unused release_date): Fixed - removed the unused RELEASE_DATE / release_date variable.

Comment 5 (doc wording mismatch): Fixed - updated wording in both the YAML workflow input description and doc/release_process.md to accurately describe auto-increment behavior for both stable and pre-release versions.