[Bugfix][Reasoning] Properly detect reasoning end when using thinking_token_budget

[schoennenbeck]

Fixes issue: #39697

When setting a thinking_token_budget there is currently a missmatch between the reasoning parser and budget enforcer considering what it means to be in thinking mode. Specifically the budget enforcer is looking for its full reasoning_end_str in the output. However, this string might include a transition phase or otherwise differ from the end string of the reasoning parser. In this case the budget enforcer does not notice when reasoning ends naturally (i.e. reasoning ended before exceeding the budget). This leads to the reasoning_end_str being forcibly added to the output when the model is already producing content and generally undesirable behaviour.

This PR fixes this behaviour by making the reasoning parsers end string (if it exists) available to the budget enforcer and using it to determine if thinking has ended while still injecting the full configured reasoning_end_str if the budget is exceeded. It also adds a sanity check to make sure that the reasoning_end_str will actually be recognized by the parser as an end to the reasoning.

Disclosure

This PR was authored with the help of Claude Code (Opus 4.7). All code has been checked by me.

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to distinguish between a user-configured reasoning_end_str and a reasoning parser's intrinsic end marker. It adds parser_reasoning_end_token_ids to ReasoningConfig and updates ThinkingBudgetStateHolder to use these IDs for detecting the end of reasoning. Validation logic is also added to ensure that the configured end string contains the parser's intrinsic marker. A typo was identified in a ValueError message where 'overrun' and 'Ensure' were concatenated without a space.