Dual-mode transaction and helper spec coverage

Audit of the transaction and helper specs for behaviors covered in only
one mode, agent or collector, that should be covered in both. Most gaps
were agent-only leftovers that assert through `to_h`-based matchers,
which do nothing in collector mode.

Fold the two send_error block specs into one dual-moded spec. It sets an
active transaction, calls `send_error` with a block, and checks the
block's metadata and the error land on `send_error`'s own transaction
while the active one is restored and left untouched. In collector mode
this confirms the active trace continues rather than being dropped or
left open.

Cover more behaviors in both modes: the `set_error` and `report_error`
blocks, `set_empty_params!`, the `add_params`, `add_session_data`,
`add_headers` and `add_custom_data` merge and block variants, `set_action`
and `set_namespace` setting the value, `report_error` not completing the
active transaction, and a transaction that reports a cause-less error
after one with causes.

Also add the one missing collector metric-coercion case, a symbol metric
name, to the OpenTelemetry metrics backend spec.

Author: unflxw

spec/lib/appsignal/metrics/opentelemetry_backend_spec.rb

@@ -46,6 +46,14 @@ def snapshot_for(name)
       snapshot = snapshot_for("my_gauge")
       expect(snapshot.data_points.first.value).to eq(10.0)
     end
+
+    it "coerces a symbol metric name to a string" do
+      described_class.set_gauge(:my_gauge, 1.0, {})
+
+      # snapshot_for matches on the string name, so this only finds the
+      # snapshot if the symbol name was coerced.
+      expect(snapshot_for("my_gauge")).not_to be_nil
+    end
   end
 
   describe ".increment_counter" do

spec/lib/appsignal/transaction_spec.rb

@@ -3943,7 +3943,8 @@ def exception_event
         )
       end
 
-      it "does not keep error causes from previously set errors" do
+      it "does not keep error causes from previously set errors", :agent_mode do
+        start_agent(**start_agent_args)
         transaction.send(:_set_error, error)
         transaction.send(:_set_error, error_without_cause)
 
@@ -3956,6 +3957,24 @@ def exception_event
         expect(transaction).to include_error_causes([])
       end
 
+      it "records no causes for a later cause-less error in collector mode",
+        :collector_mode do
+        start_collector_agent
+        transaction.send(:_set_error, error)
+        transaction.send(:_set_error, error_without_cause)
+        transaction.complete
+
+        # Collector mode records each error as its own exception event rather
+        # than overwriting, so the later cause-less error's event carries no
+        # causes; the earlier error's causes do not leak onto it.
+        events = Array(root_span.events).select { |event| event.name == "exception" }
+        without_cause = events.find do |event|
+          event.attributes["exception.message"] == "error without cause"
+        end
+        expect(without_cause).not_to be_nil
+        expect(without_cause.attributes).not_to have_key("appsignal.error_causes")
+      end
+
       describe "with app paths" do
         let(:root_path) { project_fixture_path }
         let(:error) do