server: fix json_schema response_format handling

[Copilot]

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Merges upstream ggml-org/llama.cpp#18963.

When response_format.type is "json_schema" but json_schema.schema is missing, the server silently falls back to an empty schema {} which matches any JSON — effectively no constraint at all.

Changes:

• tools/server/server-common.cpp: Validate that json_schema.schema is present; throw if missing. Update the invalid-type error message to list "json_schema" as a valid option.
• tools/server/tests/unit/test_chat_completion.py: Add test cases for OpenAI-style json_schema with extra fields (name, strict), missing schema field (expect 400), and error message content assertions.

// Before: silently accepts, produces unconstrained JSON
{"type": "json_schema", "json_schema": {"name": "test"}}

// After: returns 400 with clear error
// "response_format type \"json_schema\" requires \"json_schema.schema\" to be set"

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