Improve shell command message when agent is empty

[deleSerna]

Fixes #1741
Most of the changes are taken from #1786 (Contributed by @simeshev)

Changes:

• Added an explicit empty-state response for the agents shell command and preserve detailed output + a concise summary when agents exist.
• Improved agent registration failure warnings.
• Adjusted logging severity when an agent process runs with no goals (where goals are required).

Test

• Added tests in ShellCommandsAgentsTest
• Tested on a real agent application with embabel.agent.api.validation.manager.skip-agent-deployment-on-error=true and agent did not have any actions/goal/conditions and No agents registered displayed as expected.

   00:06:01.831 [main] WARN  AgentMetadataReader - No @EmbabelComponent or @Agent annotation found on org.springframework.shell.boot.TerminalUIAutoConfiguration
00:06:01.831 [main] WARN  AgentMetadataReader - No @EmbabelComponent or @Agent annotation found on org.springframework.shell.component.ViewComponentExecutor
00:06:01.831 [main] INFO  DelegatingAgentScanningBeanPostProcessor - All deferred beans were post-processed.
00:06:01.836 [main] INFO  Agent101Application - Started Agent101Application in 1.241 seconds (process running for 1.362)
embabel> 
<===<====No agents registeredTING [29s]
embabel> n
<===========--> 88% EXECUTING [1m 1s]

[deleSerna]

@igordayen @simeshev ready for review

[igordayen]

@deleSerna - looks good, but please verify semantic of "Agentic Component".

Also could you please confirm the level of testing outside unit testing, thanks

[deleSerna]

but please verify semantic of "Agentic Component".

Will do

Also could you please confirm the level of testing outside unit testing, thanks

As mentioned din the description, tested on a real agent application with embabel.agent.api.validation.manager.skip-agent-deployment-on-error=true and agent did not have any actions/goal/conditions and No agents registered displayed as expected.

[igordayen]

@deleSerna - looks good to me, thank you