feat!: Wire through parallel_tool_calls to Responses API (#4124)

# What does this PR do?
Initial PR against #4123
Adds `parallel_tool_calls` spec to Responses API and basic initial
implementation where no more than one function call is generated when
set to `False`.

## Test Plan
* Unit tests have been added to verify no more than one function call is
generated.
* A followup PR will verify passing through `parallel_tool_calls` to
providers.
* A followup PR will address verification and/or implementation of
incremental function calling across multiple conversational turns.

---------

Signed-off-by: Anastas Stoyanovsky <astoyano@redhat.com>

docs/static/deprecated-llama-stack-spec.yaml

@@ -3566,9 +3566,10 @@ components:
           type: array
           title: Output
         parallel_tool_calls:
-          type: boolean
-          title: Parallel Tool Calls
-          default: false
+          anyOf:
+          - type: boolean
+          - type: 'null'
+          default: true
         previous_response_id:
           anyOf:
           - type: string
@@ -3968,6 +3969,11 @@ components:
           anyOf:
           - type: string
           - type: 'null'
+        parallel_tool_calls:
+          anyOf:
+          - type: boolean
+          - type: 'null'
+          default: true
         previous_response_id:
           anyOf:
           - type: string
@@ -4094,9 +4100,10 @@ components:
           type: array
           title: Output
         parallel_tool_calls:
-          type: boolean
-          title: Parallel Tool Calls
-          default: false
+          anyOf:
+          - type: boolean
+          - type: 'null'
+          default: true
         previous_response_id:
           anyOf:
           - type: string