feat: add release candidate (RC) support to release tooling (#23600)

This adds full RC release support to the release scripts and GitHub
Actions workflow. Previously, the tooling only supported stable and
mainline releases with strict vMAJOR.MINOR.PATCH semver tags.

Changes:
- scripts/releaser/version.go: Add Pre field to version struct for
prerelease suffixes (e.g. "rc.0"), update regex, parsing, String(),
comparison methods, and add IsRC()/rcNumber() helpers.
- scripts/releaser/release.go: Detect RC branches (release/X.Y-rc.N),
suggest RC version numbers, auto-set "rc" channel (skipping
stable/mainline prompt), add RC advisory to release notes, skip docs
update for RC releases.
- .github/workflows/release.yaml: Add "rc" channel option, fix branch
derivation for RC tags (v2.32.0-rc.0 -> release/2.32-rc.0 instead of
broken release/2.32.0-rc), skip homebrew/winget/package publishing for
RC releases.
- scripts/release/publish.sh: Add --rc flag, pass --prerelease to gh
release create for RC releases.
- scripts/releaser/version_test.go: Add comprehensive unit tests for
version parsing, string formatting, IsRC, rcNumber, GreaterThan, and
Equal with RC versions.

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.github/workflows/release.yaml

@@ -9,6 +9,7 @@ on:
         options:
           - mainline
           - stable
+          - rc
       release_notes:
         description: Release notes for the publishing the release. This is required to create a release.
       dry_run:
@@ -119,9 +120,19 @@ jobs:
             exit 1
           fi
 
-          # 2.10.2 -> release/2.10
+          # Derive the release branch from the version tag.
+          # Standard: 2.10.2 -> release/2.10
+          # RC:       2.32.0-rc.0 -> release/2.32-rc.0
           version="$(./scripts/version.sh)"
-          release_branch=release/${version%.*}
+          if [[ "$version" == *-rc.* ]]; then
+            # Extract major.minor and rc suffix from e.g. 2.32.0-rc.0
+            base_version="${version%%-rc.*}"   # 2.32.0
+            major_minor="${base_version%.*}"   # 2.32
+            rc_suffix="${version##*-rc.}"      # 0
+            release_branch="release/${major_minor}-rc.${rc_suffix}"
+          else
+            release_branch=release/${version%.*}
+          fi
           branch_contains_tag=$(git branch --remotes --contains "${GITHUB_REF}" --list "*/${release_branch}" --format='%(refname)')
           if [[ -z "${branch_contains_tag}" ]]; then
             echo "Ref tag must exist in a branch named ${release_branch} when creating a release, did you use scripts/release.sh?"
@@ -531,6 +542,9 @@ jobs:
           if [[ $CODER_RELEASE_CHANNEL == "stable" ]]; then
             publish_args+=(--stable)
           fi
+          if [[ $CODER_RELEASE_CHANNEL == "rc" ]]; then
+            publish_args+=(--rc)
+          fi
           if [[ $CODER_DRY_RUN == *t* ]]; then
             publish_args+=(--dry-run)
           fi
@@ -643,7 +657,7 @@ jobs:
           retention-days: 7
 
       - name: Send repository-dispatch event
-        if: ${{ !inputs.dry_run }}
+        if: ${{ !inputs.dry_run && inputs.release_channel != 'rc' }}
         uses: peter-evans/repository-dispatch@28959ce8df70de7be546dd1250a005dd32156697 # v4.0.1
         with:
           token: ${{ secrets.CDRCI_GITHUB_TOKEN }}
@@ -731,7 +745,7 @@ jobs:
     name: Publish to winget-pkgs
     runs-on: windows-latest
     needs: release
-    if: ${{ !inputs.dry_run }}
+    if: ${{ !inputs.dry_run && inputs.release_channel != 'rc' }}
 
     steps:
       - name: Harden Runner