feat: support opening agents in MPD projects W-17961897

[cristiand391]

What issues does this PR fix or reference?

Updates the Open an agent in default org command to list all agents in the project dirs (defined in sfdx-project)

instead of just the ones in the default project dir.
DEPENDS ON: forcedotcom/agents#63

Improve error if no agents are found locally

before:

after

sets quickpick placeholder instead of title

https://code.visualstudio.com/api/ux-guidelines/quick-picks

❌ Don't
...
Use a title when the placeholder can describe the purpose on its own
...

@W-17961897@

Testing Setup Notes

1. checkout this PR and bump @salesforce/agents-bundle@dev to get these changes: feat: list local agents W-17961897 agents#63
2. compile
3. try the open agent command in projects with/without agents.

coral-cloud is an MPD project with agents: https://github.com/trailheadapps/coral-cloud

TODO:
DONE ✅
from Gordon:

Error should also be sent to the output channel for the extension. See the ChannelService and usage for examples https://github.com/forcedotcom/salesforcedx-vscode-einstein-gpt/blob/a56e3ae2912586e357f6f9f2b4d0a5f2520b2cd2/src/services/CoreExtensionService.ts#L109

[WillieRuemmele]

QA Notes

in pjson
"@salesforce/agents-bundle": "0.12.13-dev.1 ",

---

❌ : run the command, either context menu / command palette => select agent => Cannot read properties of null (reading 'toString')

✅ : no agents in the project, error pop up Could not find agents in the current project.