Python: Json schema improvements for Union and Optional

[moonbox3]

Motivation and Context

We have some gaps in how we build the JSON schema for function calling related to Union and Optional types. There is also some work to do related to handling Optional versus param | None = None.

Description

This PR improves the JSON schema building for the Union and Optional types. It adds additional tests and improves some existing tests.

• Closes Python: Bug: Kernel functions cannot have optional arguments. #7297
• Closes Python: Bug: Kernel functions do not handle Union arguments well #7298

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Python 3.10 Test Coverage Report

File	Stmts	Miss	Cover	Missing
semantic_kernel
kernel.py	157	9	94%	145, 156, 160, 200–201, 310–313, 422
semantic_kernel/connectors
telemetry.py	16	2	88%	18–19
semantic_kernel/connectors/ai/azure_ai_inference/services
azure_ai_inference_chat_completion.py	149	4	97%	169, 312, 416, 430
semantic_kernel/connectors/ai/embeddings
embedding_generator_base.py	8	1	88%	50
semantic_kernel/connectors/ai/hugging_face/services
hf_text_completion.py	54	3	94%	95, 103, 134
hf_text_embedding.py	32	5	84%	77–81
semantic_kernel/connectors/openapi_plugin
openapi_manager.py	58	2	97%	110–111
openapi_parser.py	88	1	99%	127
openapi_runner.py	105	2	98%	181–182
semantic_kernel/connectors/openapi_plugin/models
rest_api_operation.py	129	1	99%	240
semantic_kernel/functions
kernel_function_decorator.py	98	1	99%	102
kernel_function_from_method.py	96	1	99%	153
kernel_function_from_prompt.py	154	7	95%	165–166, 180, 200, 218, 238, 321
kernel_plugin.py	187	2	99%	472, 475
semantic_kernel/planners
plan.py	234	45	81%	52, 161–163, 195, 212–225, 262, 267, 275–276, 286–289, 306, 311, 327, 330–335, 353, 358, 361, 363, 370, 384–386, 391–395
semantic_kernel/planners/function_calling_stepwise_planner
function_calling_stepwise_planner.py	116	4	97%	143, 187–188, 196
semantic_kernel/planners/sequential_planner
sequential_planner.py	64	6	91%	69, 73, 107, 123, 132–133
sequential_planner_extensions.py	50	9	82%	29–30, 52, 106–120
sequential_planner_parser.py	77	12	84%	64–72, 91, 115–118
semantic_kernel/schema
kernel_json_schema_builder.py	119	5	96%	48, 82, 204, 208–209
semantic_kernel/template_engine/blocks
code_block.py	77	1	99%	119
named_arg_block.py	43	1	98%	98
TOTAL	7061	124	98%

Python 3.10 Unit Test Overview

Tests	Skipped	Failures	Errors	Time
1903	1 💤	0 ❌	0 🔥	32.165s ⏱️