Update docs build to tolerate tags that have been pushed but not yet built and published

[wilkinsona]

The v4.0.2 tag has been pushed but the resulting spring-boot-docs-4.0.2-root-aggregate-content.zip artifact has not yet been built and published. This breaks the docs build:

Run node run.js --no-checkout
  node run.js --no-checkout
  shell: /usr/bin/bash -e {0}
  env:
    ARTIFACTORY_USERNAME: ***
    ARTIFACTORY_PASSWORD: ***
    COMMERCIAL_REPO_USERNAME: 
    COMMERCIAL_REPO_PASSWORD: 
    COMMERCIAL_SNAPSHOT_REPO_URL: 
    COMMERCIAL_RELEASE_REPO_URL: 
    BUILD_REFNAME: 3.5.x
    BUILD_VERSION: 3.5.11-SNAPSHOT
Installing modules
Running Antora
$ npx antora antora-playbook.yml --stacktrace --ui-bundle-url https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.25/ui-bundle.zip
partial-build-extension: version 3.5.11 not previously built; reverting to full build
[12:44:03.539] FATAL (antora): Unable to download 'https://repo.spring.io/release/org/springframework/boot/spring-boot-docs/4.0.2/spring-boot-docs-4.0.2-root-aggregate-content.zip' due to HTTP response code 404 ()
    Cause: HTTPError
        at download (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:409:27)
        at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
        at async getZipFile (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:292:20)
        at async doWithIncludes (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:251:23)
        at async addContentAggregateIncludes (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:115:7)
        at async GeneratorContext.contentAggregated (/home/runner/work/spring-boot/spring-boot/node_modules/@springio/antora-zip-contents-collector-extension/lib/index.js:50:11)
        at async GeneratorContext.notify (/home/runner/work/spring-boot/spring-boot/node_modules/@antora/site-generator/lib/generator-context.js:66:39)
        at async Promise.all (index 0)
        at async generateSite (/home/runner/work/spring-boot/spring-boot/node_modules/@antora/site-generator/lib/generate-site.js:25:5)
        at async Command.parseAsync (/home/runner/work/spring-boot/spring-boot/node_modules/commander/lib/command.js:1092:5)
Unexpected error

It would be better if it somehow ignored the tag instead.

[dlwldnjs1009]

Hi @wilkinsona, I'd like to take a crack at this.

My approach would be to check whether the docs zip artifact actually exists
(e.g., HEAD request to the artifact URL) before including each tag in the build.
Tags returning 404 would simply be skipped, allowing the build to proceed
gracefully without failing.

Does this align with what you had in mind, or would you prefer a different
approach—such as handling this in the zip-contents-collector extension itself?

[wilkinsona]

I'm not sure yet. This will almost certainly have to be tackled somewhere outside of this repository. The issue is largely a reminder that we need someone on the team to look at it.