fix(function_call): skip undefined tools in DeepSeek V3.2/V4 streams - #1727

DeepSeek V3.2/V4 can emit a streamed call to an undefined tool when its DSML invoke closes, as noted in #1697. For a request defining only get_weather, an invoke for undefined_tool becomes a tool call in the response.

Validate the name before allocating call state in both streaming branches. Skip undefined invokes and continue parsing the buffer so subsequent valid calls keep their arguments and indices.

The 36 new regression cases cover both parsers, chunk boundaries, and undefined calls before, between and after valid calls. Ran the same 85 tests, including the new cases, against the base commit (1a2a306d) and this change on Linux with Python 3.10:

python -m pytest -q \
  unit_tests/server/test_deepseek_tool_parser_stream.py \
  unit_tests/server/test_deepseek_dsml_stream_args.py \
  unit_tests/server/test_function_call_common_prefix.py \
  unit_tests/server/test_glm47_tool_parser.py \
  unit_tests/server/test_qwen3_coder_stream_fc.py