chore: prepare release workflow for codegen 3.0

[ewaostrowska]

A manual pre-release workflow for swagger-codegen 3.0 that prepares a release PR against 3.0.0 with validated version inputs and synchronized release file updates.

What’s included

1. Added GitHub Actions workflow: .github/workflows/prepare-release-3.yml
2. Added shared release helpers: CI/release/common.sh
3. Added release preparation script: CI/release/prepare-codegen-release.sh
4. Added release file updater: CI/release/update-codegen-release-files.py

Workflow behavior

1. Runs manually via workflow_dispatch.
2. Resolves/validates release inputs (codegen_version, next snapshot, generators settings).
3. Verifies generator artifact availability (release or snapshot).
4. Updates project/release files (POMs, docs, OpenAPI metadata, release notes draft).
5. Builds release candidate with explicit generators version.
6. Opens a PR to 3.0.0 from prepare-release-.

Inputs in prepare-release-3

1. codegen_version
   Release version to prepare (e.g. 3.0.80). If empty, derived from current root -SNAPSHOT.
2. next_codegen_snapshot_version
   Next dev version after release (e.g. 3.0.81-SNAPSHOT). If empty, auto-incremented from codegen_version.
3. release_generators
   true/false switch for whether this release is coordinated with generators release flow.
4. generators_version
   Generators release version to set in codegen when needed (e.g. 1.0.61).
5. previous_generators_version
   Optional bootstrap version used for dependency-cycle-safe validation/build (can be release or -SNAPSHOT).

[Copilot]

Pull request overview

Adds a manual GitHub Actions “prepare release” workflow for the Swagger Codegen 3.0 line, plus supporting release helper scripts to validate version inputs, update release-related files (POMs/docs/OpenAPI metadata), build a release candidate, and open a PR against the 3.0.0 branch.

Changes:

• Introduces a workflow_dispatch GitHub Actions workflow to prepare a 3.0 release PR.
• Adds shared Bash release helpers for version validation and artifact existence checks against Maven Central / Sonatype snapshots.
• Adds scripts to bump the project version, generate release notes draft, and update release references across docs/POMs/OpenAPI metadata.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
.github/workflows/prepare-release-3.yml	Manual workflow to prepare a 3.0.x release branch/PR, validate inputs, build with pinned generators, and open a PR.
CI/release/common.sh	Shared Bash helpers for version parsing/validation and Maven artifact probing.
CI/release/prepare-codegen-release.sh	Main “prepare release” script: resolves versions, bumps Maven version, drafts release notes, and runs file updates.
CI/release/update-codegen-release-files.py	Updates docs/POMs/OpenAPI metadata to reflect the intended release + next snapshot versions.

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