common: load parser in common_chat_parser_params constructor

[sacredvoid]

Overview

common_chat_parser_params copies format and generation_prompt from common_chat_params but never calls parser.load(). Without that call, common_chat_parse() falls back to a content-only parser, so tool calls end up in msg.content instead of msg.tool_calls. The server works around this by loading the parser separately, but anyone using the library directly hits this bug.

Adds the missing parser.load() call in the constructor (3 lines) and a test that verifies the constructor path parses tool calls end-to-end.

Additional information

Fixes #21650

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES - used for identifying the root cause and drafting the fix + test.