Add GA headers to HTTP responses, add live-check fixtures as site test pages

[chalin]

• Contributes to Address site observability and data access #9559
• Adds X-Asset-Fetch-Ga-Info response headers to schema, negotiated Markdown, and direct asset responses
• Updates the asset-fetch analytics plan and edit history to match the current implementation
• Uses ga-event-candidate with config-present or config-missing to expose derived GA path and coarse emission state
• Keeps non-candidate cases readable with none: <reason> header values
• Extends unit and cross-function integration tests to validate the new header behavior
• Splits deployed-host live checks so delivery assertions and header assertions stay separate
• Adds shared edge-function test helpers and makes the test suites more DRY without changing coverage
• Splits deployed-host live checks so delivery assertions and GA-info header assertions stay separate
• Adds isolated /site/testing/tests/ fixtures, including a regular page and an HTML-only page, for low-impact live checks

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All unit, integration, and live tests are passing.

New response header is being generated, e.g.:

$ curl -sI  -H 'Accept: text/markdown' https://deploy-preview-9630--opentelemetry.netlify.app/site/testing/tests/regular/
HTTP/2 200 
...
x-asset-fetch-ga-info: /site/testing/tests/regular/index.md;ga-event-candidate,config-missing
...