fix: use time.Equal() for external auth token expiry comparison (#22295)

The listen loop in workspaceAgentsExternalAuthListen compared
OAuthExpiry using == which compares `time.Time` internal struct fields
including the `*time.Location` pointer.

`time.LoadLocation` does not cache the returned `*Location` pointer, so
each lib/pq connection gets a distinct pointer for the same timezone.
When `pq.ParseTimestamp()` applies the connection's location to a parsed
timestamp, the resulting time.Time embeds that connection-specific
pointer. If the `sql.DB` pool hands out different connections for the
two GetExternalAuthLink reads, the identical timestamp produces
`time.Time` values where == returns false despite representing the same
instant. This is intermittent because the pool _usually_ reuses the same
connection for sequential queries.

This change uses `.Equal()` to compare instants regardless of location.
Also makes the test's validation call counter atomic to fix a possible
data race between the HTTP server and test goroutines.

coderd/workspaceagents.go

@@ -2045,7 +2045,7 @@ func (api *API) workspaceAgentsExternalAuthListen(ctx context.Context, rw http.R
 		// No point in trying to validate the same token over and over again.
 		if previousToken.OAuthAccessToken == externalAuthLink.OAuthAccessToken &&
 			previousToken.OAuthRefreshToken == externalAuthLink.OAuthRefreshToken &&
-			previousToken.OAuthExpiry == externalAuthLink.OAuthExpiry {
+			previousToken.OAuthExpiry.Equal(externalAuthLink.OAuthExpiry) {
 			continue
 		}