fix: handle agent not found

[mattjohnsonpint]

Add proper error handling for checking when an agent id is not found

[Copilot]

Pull Request Overview

This PR adds proper error handling for cases where an agent ID is not found in the database. The changes standardize error handling across both ModusDB and PostgreSQL database backends by introducing a specific error type and consistent error checking patterns.

• Introduces a new ErrAgentNotFound error variable for consistent error handling
• Updates both database backend functions to return the specific error when no agent is found
• Replaces nil state checks with proper error type checking in the agent message handling logic

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
runtime/db/agentstate.go	Adds ErrAgentNotFound variable and updates both database functions to return this error when agent is not found
runtime/actors/agents.go	Replaces nil state check with proper error type checking using ErrAgentNotFound
CHANGELOG.md	Documents the bug fix in the changelog

Comments suppressed due to low confidence (1)

runtime/actors/agents.go:228

• [nitpick] The error message format is inconsistent. The previous message was "agent not found" but now includes the agent ID. Consider using a consistent format like "agent not found" or always include the ID for better debugging.

				return newAgentMessageErrorResponse(fmt.Sprintf("agent %s not found", agentId)), nil