Fix Envoy inventory archive URL and HTTP status handling

[Copilot]

project sync requests inventories from a nonexistent GCS bucket, leaving newer releases without inventories. It also accepts non-404 error responses as inventory content.

• Archive URL: Use envoy-cncf-archive; preserve the /envoy/docs/v{version}/objects.inv path and ENVOY_DOCS_ARCHIVE_URL override.
• Fetch handling: Accept only HTTP 200. Other statuses fall through to the legacy URL, returning None if neither succeeds.
• Regression coverage: Update the expected URL and cover 200, 301, 403, 404, and 500 for both sources. Leave the package version unchanged.

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