Add more guardrails based on agent types; onboard agentic_context processor

[ohltyler]

Description

Misc enhancements:

• removes the 'optional' field for memory, and sets the default memory configuration when creating a new conversational agent, as such it's impossible to create a new conversational agent without memory configured now.
• removes the "All indices" option in the index selection for flow agents. If a flow agent is selected, users must select an index. If a conversational index is selected, it is defaulted to "all indices". If a user switches from flow -> conversational, the selected index remains the same. If a user switches from conversational -> flow, and if "all indices" was selected, it is set back to blank/empty.
• onboards the agentic_context search response processor, which propagates agent responses from the query planner tool (the DSL query, the agent summary/reasoning), into ext field to pass back to the user

Also:

• adds back "no selected index" error state, if no index selected and the agent type is flow
• adds loading state while fetching the agent details to dynamically render test flow panel (due to differences in agent dropdown)

Check List

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.