[PLA-2420] Fix release CI

[JayPavlina]

• change default user agent to Enjin-Platform-CSharp-SDK/{version}
• set default graphql endpoint to https://platform.enjin.io/graphql
• note this requires either setting release to draft, or creating the tag and having CI create the release

Other changes:
This pull request consolidates the Unity Package Manager (UPM) build and release process into the main build.yml workflow, eliminating the separate unity.yml workflow. It introduces a new upm job and a publish-release job to automate UPM packaging, tagging, and publishing alongside NuGet package publishing. The workflow now supports both tag-based and manual (workflow_dispatch) triggers, and ensures that all release assets are attached to a single draft release before publication.

Key changes include:

Workflow consolidation and triggers:

• Removed the separate .github/workflows/unity.yml file, moving all UPM-related build and release steps into .github/workflows/build.yml for a unified release process.
• Added a workflow_dispatch trigger with a version input to build.yml, allowing manual builds for specific UPM versions.

UPM packaging and release automation:

• Introduced a new upm job in build.yml that builds the SDK, assembles the UPM package, pushes a upm/v<version> tag, and attaches the UPM tarball to the draft GitHub Release. This job runs on tag pushes or manual dispatch.
• Added a publish-release job that publishes the GitHub Release (removes draft status) only after both NuGet and UPM assets are attached, ensuring atomic release publication.

NuGet publishing improvements:

• Updated the NuGet package upload step to always create a draft release on tag pushes, avoiding conflicts when multiple assets are uploaded, and added a step to publish packages to NuGet.org.

[JayPavlina]

Needs a few things before merge:

• Tag ruleset— update it to allow upm/v* tags (or add a bypass for github-actions). The workflow comment near thePush upm/ tagstep references this. (or we could change name format)
• Add the NUGET_API_KEY secret in repo Settings → Secrets and variables → Actions. Use an API key from nuget.org scoped to the Enjin.Platform.Sdk package.
• v3.0.0 release and tag should be deleted
• should do a dry run of the workflow on this branch (of parts that can be tested)

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Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[JayPavlina]

This one needs to be addressed, but I don't know what the final name of the tag will be yet