Python: Respect optional MCP prompt arguments (#14676)

### Motivation and Context

MCP prompts are loaded with every argument marked required, ignoring
`PromptArgument.required` (optional; absent means not required). The
function-calling schema then lists all of them as required, and invoking
the prompt without an optional argument fails with "Parameter style is
required but not provided in the arguments."

### Description

Use `bool(prompt_argument.required)`.

- New test: required/optional/absent arguments map to True/False/False,
and invoking with only the required one sends `arguments={"code": ...}`;
fails before, passes after.
- `tests/unit/connectors/mcp`, `tests/unit/functions` and
`tests/unit/kernel` pass; ruff and mypy clean.

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: jaeminbird

python/semantic_kernel/connectors/mcp.py

@@ -195,7 +195,7 @@ def _get_parameter_dict_from_mcp_prompt(prompt: types.Prompt) -> list[dict[str,
         {
             "name": prompt_argument.name,
             "description": prompt_argument.description,
-            "is_required": True,
+            "is_required": bool(prompt_argument.required),
             "type_object": str,
         }
         for prompt_argument in prompt.arguments

python/tests/unit/connectors/mcp/test_mcp.py

@@ -900,3 +900,36 @@ def echo() -> str:
         result = await client.call_tool(expected_name, {})
         assert not result.isError
         assert result.content == [types.TextContent(type="text", text="hello")]
+
+
+async def test_mcp_prompt_optional_arguments_are_not_required():
+    plugin = MCPStdioPlugin(name="test", command="echo", load_tools=False)
+    plugin.session = AsyncMock(spec=ClientSession)
+    plugin.session.list_prompts.return_value = types.ListPromptsResult(
+        prompts=[
+            types.Prompt(
+                name="review",
+                arguments=[
+                    types.PromptArgument(name="code", required=True),
+                    types.PromptArgument(name="style", required=False),
+                    types.PromptArgument(name="language"),
+                ],
+            )
+        ]
+    )
+    plugin.session.get_prompt.return_value = types.GetPromptResult(
+        messages=[types.PromptMessage(role="user", content=types.TextContent(type="text", text="review this"))]
+    )
+    await plugin.load_prompts()
+    kernel = Kernel()
+    kernel.add_plugin(plugin, "test")
+    function = kernel.get_function("test", "review")
+
+    assert {p.name: p.is_required for p in function.metadata.parameters} == {
+        "code": True,
+        "style": False,
+        "language": False,
+    }
+    result = await kernel.invoke(function, code="x = 1")
+    assert str(result) == "review this"
+    plugin.session.get_prompt.assert_awaited_once_with("review", arguments={"code": "x = 1"})