refactor: streamline agents logic and remove unnecessary caching (#4298)

* refactor: enhance agent loading with structured data

Introduced a new struct, EnabledAgent, to encapsulate agent name and type
information (plugin or built-in). Updated the getEnabledAgentNames function
to return a slice of EnabledAgent instead of a slice of strings, allowing
for more detailed agent management. This change improves the clarity and
maintainability of the code by providing a structured approach to handling
enabled agents and their types.

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor: remove agent caching logic

Eliminated the caching mechanism for agents, including the associated
data structures and methods. This change simplifies the agent loading
process by directly retrieving agents without caching, which is no longer
necessary for the current implementation. The removal of this logic helps
reduce complexity and improve maintainability of the codebase.

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor: replace range with slice.Contains

Signed-off-by: Deluan <deluan@navidrome.org>

* test: simplify agent name extraction in tests

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/agents/agents_test.go

@@ -56,8 +56,8 @@ var _ = Describe("Agents", func() {
 
 		It("does not register disabled agents", func() {
 			var ags []string
-			for _, name := range ag.getEnabledAgentNames() {
-				agent := ag.getAgent(name)
+			for _, enabledAgent := range ag.getEnabledAgentNames() {
+				agent := ag.getAgent(enabledAgent)
 				if agent != nil {
 					ags = append(ags, agent.AgentName())
 				}