fix(tui): don't crash when there are MCP servers but no agents

[iainlane]

When there are MCP servers but no AI agents configured, the TUI currently crashes due to a nil pointer dereference. We receive an mcp.EventStateChanged event, when the MCP server is initialised. This causes us to attempt to access the agent coordinator, which is nil when there are no agents set up.

The fix is to add a nil guard before accessing the coordinator. With this fix, instead of crashing, the app proceeds to the UI, allowing credentials to be set up as normal.

Tests are included. Reverting the fix causes the tests to fail as expected.

• [x] I have read CONTRIBUTING.md.

[charmcli]

All contributors have signed the CLA ✍️ ✅
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[iainlane]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[LarsArtmann]

Do we need any UI feedback? I tend to say no, but I rather ask.