fix: cache responses without public directive

[fiam]

@coderabbitai summary

Summary

• Cache responses with a positive max-age or s-maxage even when public is absent.
• Apply the configured fallback TTL when a response supplies recognized caching intent without an explicit freshness lifetime.
• Keep missing, empty, and extension-only Cache-Control headers from opting responses into caching.
• Preserve no-store, no-cache, private, invalid, and zero-TTL safeguards.
• Expand unit, parser, and resolver-level coverage for cache-control behavior and precedence.

RFC 9111 section 5.2.2.9 states that public is unnecessary when a response is already cacheable: https://www.rfc-editor.org/rfc/rfc9111.html#section-5.2.2.9

Linear: https://linear.app/wundergraph/issue/ENG-9971/when-public-is-not-present-we-should-still-not-skip-caching

Testing

• go test ./...
• go test -race ./pkg/caching ./pkg/engine/cache

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.

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