fix(coderd): render collected_at as UTC RFC3339 in the agent metadata aggregate (#27991)

Follow-up to #27934; this fix was pushed to the branch after the
squash-merge and missed it.

`jsonb_build_object` renders timestamptz in the session `TimeZone`,
which Coder never pins, and `collected_at` defaults to year 1 until the
agent's first report. On a non-UTC Postgres session a
registered-but-never-collected item renders with an LMT second-offset
(even `BC`, e.g. `0001-12-31T19:03:58-04:56:02 BC`), which Go's RFC3339
parsing rejects - a 500 for the entire list page whenever
`include_agent_metadata` is used.

- `to_char(... AT TIME ZONE 'UTC', ...)` pins the rendering;
never-collected items round-trip as Go's zero time.
- The test now runs against a named-zone database
(`dbtestutil.WithTimezone("America/Caracas")`) and requests a registered
but never-collected key; it reproduces the 500 without the fix.

Also contains the failure mode Go-side: an unparsable aggregate now
degrades to missing metadata for that workspace (with a warning log)
instead of failing the entire page. The SQL fix prevents the known
cause; the containment covers any future one. The test still catches
regressions because it asserts the metadata values, not just a 200.

---

Authored by Coder Agents on behalf of @Emyrk.

coderd/database/queries/workspaces.sql

@@ -494,7 +494,11 @@ SELECT
 					'error', workspace_agent_metadata.error,
 					'timeout', workspace_agent_metadata.timeout,
 					'interval', workspace_agent_metadata.interval,
-					'collected_at', workspace_agent_metadata.collected_at,
+					-- Rendered explicitly as UTC RFC3339: jsonb renders
+					-- timestamptz in the session TimeZone, and the year-1
+					-- default of collected_at renders named zones with
+					-- LMT second-offsets (even BC), which Go rejects.
+					'collected_at', to_char(workspace_agent_metadata.collected_at AT TIME ZONE 'UTC', 'YYYY-MM-DD"T"HH24:MI:SS.US"Z"'),
 					'display_order', workspace_agent_metadata.display_order
 				))
 			FROM