fix!: enforce agent names be case-insensitive-unique per-workspace (#16614)

Relates to https://github.com/coder/coder-desktop-macos/issues/54

Currently, it's possible to have two agents within the same workspace whose names only differ in capitalization:
This leads to an ambiguity in two cases:
- For CoderVPN, we'd like to allow support to workspaces with a hostname of the form: `agent.workspace.username.coder`.
- Workspace apps (`coder_app`s) currently use subdomains of the form: `<app>--<agent>--<workspace>--<username>(--<suffix>)?`.

Of note is that DNS hosts must be strictly lower case, hence the ambiguity.

This fix is technically a breaking change, but only for the incredibly rare use case where a user has:
- A workspace with two agents
- Those agent names differ only in capitalization.

Those templates & workspaces will now fail to build. This can be fixed by choosing wholly unique names for the agents.

coderd/provisionerdserver/provisionerdserver_test.go

@@ -1905,6 +1905,32 @@ func TestInsertWorkspaceResource(t *testing.T) {
 		})
 		require.ErrorContains(t, err, "duplicate app slug")
 	})
+	t.Run("DuplicateAgentNames", func(t *testing.T) {
+		t.Parallel()
+		db := dbmem.New()
+		job := uuid.New()
+		// case-insensitive-unique
+		err := insert(db, job, &sdkproto.Resource{
+			Name: "something",
+			Type: "aws_instance",
+			Agents: []*sdkproto.Agent{{
+				Name: "dev",
+			}, {
+				Name: "Dev",
+			}},
+		})
+		require.ErrorContains(t, err, "duplicate agent name")
+		err = insert(db, job, &sdkproto.Resource{
+			Name: "something",
+			Type: "aws_instance",
+			Agents: []*sdkproto.Agent{{
+				Name: "dev",
+			}, {
+				Name: "dev",
+			}},
+		})
+		require.ErrorContains(t, err, "duplicate agent name")
+	})
 	t.Run("Success", func(t *testing.T) {
 		t.Parallel()
 		db := dbmem.New()