feat: Enhance memory system with isolated memory configuration (#2597)

* feat: support defining any memory in an isolated way

This change makes it easier to use a specific memory type without unintentionally enabling all others.

Previously, setting memory=True would implicitly configure all available memories (like LTM and STM), which might not be ideal in all cases. For example, when building a chatbot that only needs an external memory, users were forced to also configure LTM and STM — which rely on default OpenAPI embeddings — even if they weren’t needed.

With this update, users can now define a single memory in isolation, making the configuration process simpler and more flexible.

* feat: add tests to ensure we are able to use contextual memory by set individual memories

* docs: enhance memory documentation

* feat: warn when long-term memory is defined but entity memory is not

docs/concepts/memory.mdx

@@ -145,6 +145,7 @@ from crewai.memory import LongTermMemory
 # Simple memory configuration
 crew = Crew(memory=True)  # Uses default storage locations
 ```
+Note that External Memory won’t be defined when `memory=True` is set, as we can’t infer which external memory would be suitable for your case
 
 ### Custom Storage Configuration
 ```python
@@ -278,15 +279,19 @@ crew = Crew(
 ### Using External Memory
 
 External Memory is a powerful feature that allows you to integrate external memory systems with your CrewAI applications. This is particularly useful when you want to use specialized memory providers or maintain memory across different applications.
+Since it’s an external memory, we’re not able to add a default value to it - unlike with Long Term and Short Term memory. 
 
 #### Basic Usage with Mem0
 
 The most common way to use External Memory is with Mem0 as the provider:
 
 ```python
+import os
 from crewai import Agent, Crew, Process, Task
 from crewai.memory.external.external_memory import ExternalMemory
 
+os.environ["MEM0_API_KEY"] = "YOUR-API-KEY"
+
 agent = Agent(
     role="You are a helpful assistant",
     goal="Plan a vacation for the user",
@@ -304,7 +309,6 @@ crew = Crew(
     tasks=[task],
     verbose=True,
     process=Process.sequential,
-    memory=True,
     external_memory=ExternalMemory(
         embedder_config={"provider": "mem0", "config": {"user_id": "U-123"}} # you can provide an entire Mem0 configuration
     ),
@@ -363,7 +367,6 @@ crew = Crew(
     tasks=[task],
     verbose=True,
     process=Process.sequential,
-    memory=True,
     external_memory=external_memory,
 )