chat: fix handling of space in reasoning markers

[pwilkin]

Overview

Extracted from #22162 (thanks @roj234 ), just the fix for the parser

Additional information

We're putting off the prefill changes for a further PR (prepared by @aldehir ) so I'm just taking this fix as a standalone.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: No

[aldehir]

CI is mad again

[pwilkin]

Need to fix tests

[pwilkin]

Aight, fixed tests, ran server tests locally, ran a session with Qwen3.5 on OpenCode, looks good.

[aldehir]

Just need to clean up the trailing whitespace.

tests/test-chat.cpp:
	4003: Trailing whitespace

1 errors found