chore!: remove the agents (sessions and turns) API (#4055)

- Removes the deprecated agents (sessions and turns) API that was marked
alpha in 0.3.0
- Cleans up unused imports and orphaned types after the API removal
- Removes `SessionNotFoundError` and `AgentTurnInputType` which are no
longer needed

The agents API is completely superseded by the Responses + Conversations
APIs, and the client SDK Agent class already uses those implementations.

Corresponding client-side PR:
llamastack/llama-stack-client-python#295

Author: ashwinb

client-sdks/stainless/openapi.yml

@@ -8137,20 +8137,6 @@ components:
         - error
       title: ViolationLevel
       description: Severity level of a safety violation.
-    AgentTurnInputType:
-      type: object
-      properties:
-        type:
-          type: string
-          const: agent_turn_input
-          default: agent_turn_input
-          description: >-
-            Discriminator type. Always "agent_turn_input"
-      additionalProperties: false
-      required:
-        - type
-      title: AgentTurnInputType
-      description: Parameter type for agent turn input.
     AggregationFunctionType:
       type: string
       enum:
@@ -8393,7 +8379,6 @@ components:
             - $ref: '#/components/schemas/UnionType'
             - $ref: '#/components/schemas/ChatCompletionInputType'
             - $ref: '#/components/schemas/CompletionInputType'
-            - $ref: '#/components/schemas/AgentTurnInputType'
           discriminator:
             propertyName: type
             mapping:
@@ -8406,7 +8391,6 @@ components:
               union: '#/components/schemas/UnionType'
               chat_completion_input: '#/components/schemas/ChatCompletionInputType'
               completion_input: '#/components/schemas/CompletionInputType'
-              agent_turn_input: '#/components/schemas/AgentTurnInputType'
         params:
           $ref: '#/components/schemas/ScoringFnParams'
       additionalProperties: false
@@ -8487,7 +8471,6 @@ components:
         - $ref: '#/components/schemas/UnionType'
         - $ref: '#/components/schemas/ChatCompletionInputType'
         - $ref: '#/components/schemas/CompletionInputType'
-        - $ref: '#/components/schemas/AgentTurnInputType'
       discriminator:
         propertyName: type
         mapping:
@@ -8500,7 +8483,6 @@ components:
           union: '#/components/schemas/UnionType'
           chat_completion_input: '#/components/schemas/ChatCompletionInputType'
           completion_input: '#/components/schemas/CompletionInputType'
-          agent_turn_input: '#/components/schemas/AgentTurnInputType'
     RegisterScoringFunctionRequest:
       type: object
       properties: