Fix conditional GET across the API

[warwickschroeder]

ServicePulse polls data from ServiceControl as often as 5s. ServiceControl was built to answer "nothing has changed", but the ETag it sends was a bare token and RFC 9110 requires a quoted string. NotModifiedStatusHttpHandler compares typed header values, an unquoted token fails to parse as EntityTagHeaderValue and yields null, so the comparison never matched and every conditional request got a full payload.

What changed

• WithEtag quotes the validator, in ServiceControl and in ServiceControl.Audit, which has its own copy of the same code and the same defect. All 37 ETag call sites funnel through it.
• WithEtag emits no header when there is nothing to validate. An empty validator quoted becomes ETag: "", which is well formed and therefore matches itself, so quoting alone would have answered 304 to a client holding an unrelated payload.
• Two body endpoints now use WithEtag instead of assigning Response.Headers.ETag directly. Bodies are the largest payloads on the API and immutable once written.
• ScatterGatherApi reads a remote instance's ETag from the raw header. The typed accessor returns null for an unquoted value, so every remote validator was being discarded.
• The in-memory audit persister supplies a stable body validator. It returned string.Empty for a body in metadata and a fresh Guid.NewGuid() per fetch for one in an attachment. Both can never match. It now uses the body id.

Impact assessment

• Reviewed against ServiceControl iteselt, ServicePulse, and ServiceInsight. Nothing breaks.
• Mixed-version clusters are safe. Instances read each other's validators raw and accept both shapes.

Tests

• ConditionalGetTests, 7 tests in ServiceControl.UnitTests and 7 in ServiceControl.Audit.UnitTests: the 304 round trip, the mismatch guard, both validator shapes, and the empty case.
• When_a_request_is_repeated_with_its_etag, 3 cases: /api/customchecks, GET /api/redirects and HEAD /api/redirect end to end through Kestrel, including that Total-Count survives the 304. The HEAD verb is covered nowhere else.
• When_a_message_body_is_requested_twice: the audit body endpoint

[johnsimons]

LGTM
But I noticed the monitoring instance does not use etags 😞