[WEB-10045] Use platform astro-adapter-aws-lambda for CI builds

[devindford]

What does this PR do? What is the motivation?

Prepares the Astro /api app for the websites deployment platform's SSR model by building with the platform adapter in CI.

• Adds astro-adapter-aws-lambda v1.0.0 (published via corp-node-packages) as a dependency.
• astro.config.mjs now selects the adapter per build environment (resolveSiteEnv()):
  • CI builds (build:preview / build:live): use astro-adapter-aws-lambda in static: true mode. This generates dist/_routes.json (default: static, empty ssr) that the deploy pipeline turns into the CloudFront KVS manifest, replacing the hand-written one, and emits the dist/lambda/entry.mjs stub the platform build tooling expects.
  • Local builds/dev: unchanged @astrojs/node standalone, so astro preview keeps working (the platform adapter has no preview mode).
• The output: "server" + per-route prerender = true shape is unchanged; staticApiGuard still enforces the fully-static invariant. When the first SSR route lands, static flips to false with platformMode per the divergences doc checklist.

Companion PRs:

• ddoghq/documentation-ci#253 (dynamic _routes.json manifest + handler upload)
• DataDog/corp-node-packages#361 (the adapter package)

Fixes WEB-10045 (follow-up: SSR readiness)

Merge readiness

• Ready for merge

For Datadog employees:

• ⚠️ Your branch name MUST follow the <name>/<description> convention and include the forward slash (/).
• 🤖 Comment with /review to run an automated check before review.

AI assistance

pi (Claude) helped wire the adapter selection into astro.config.mjs and update package.json/yarn.lock.

Additional notes

• Verified locally: astro sync loads the config in both CI mode (with CI_ENVIRONMENT_NAME+CI_COMMIT_REF_NAME) and local mode; the adapter package resolves and instantiates.
• Unit test suite matches the pre-existing baseline exactly (7 viewsBuilder + 7 api-html-snapshots fixture-snapshot failures exist on the base branch without these changes).
• Recommended validation: run CI_ENVIRONMENT_NAME=preview CI_COMMIT_REF_NAME=<branch> yarn build:preview (or let CI run it) and confirm dist/_routes.json is emitted with "default":"static" and empty ssr, and that dist/lambda/entry.mjs is present.

[github-actions]

Preview links (active after the build_preview check completes)

No changed files under hugo/content/en/, e.g. this PR only touches the Astro API docs app:

• https://docs-staging.datadoghq.com/devin.ford/astro-platform-adapter-v2/api/latest/