Support invocations_ws in agent init

[v1212]

Summary

• Adds invocations_ws as a selectable protocol for azd ai agent init.
• Keeps the no-prompt/default protocol as responses and documents invocations_ws in agent help/schema descriptions.
• Adds regression coverage for websocket protocol selection and code deploy serialization via protocol_versions.

Validation

• go test ./... from cli/azd/extensions/azure.ai.agents
• go build ./... from cli/azd/extensions/azure.ai.agents
• azd x build from cli/azd/extensions/azure.ai.agents
• PR CI/checks are green.

Live validation

• Local extension build installed successfully.
• azd ai agent init --protocol invocations_ws is accepted and reaches template selection, confirming protocol parsing no longer rejects it.
• Direct code deploy validation succeeded against a North Central US Foundry project in ActivityProtocol-Dev:
  • Account/project: voiceazd0708acct / voiceazd0708proj
  • Agent: voice-ws-echo:1
  • Uploaded metadata contains definition.protocol_versions: [{ protocol: invocations_ws, version: 2.0.0 }]
  • Agent reached active
  • Direct invocations_ws smoke passed: received session_started, 9600 audio bytes, response_done, no error.
• Full azd deploy validation is still blocked locally by azd auth: this machine's az CLI is logged in and can deploy via direct API, but azd deploy requires azd auth login; I did not complete an interactive azd device login in order to avoid disrupting other local work.