Add Olmo 3 reasoning parser

[soldni]

Purpose

This PR adds support for parsing reasoning traces for the Olmo 3 family of models. (support for models has been already merged in #24534).

Olmo models use <think> and </think> string to bracket their reasoning traces; however, unlike Qwen or Deepseek families, the strings are not special tokens in the vocabulary, therefore requiring a new parser.

Test Plan

This PR includes tests to verify that the parser behaves as expected. Existing tests should not be impacted.

Test Result

Run tests:

pytest tests/reasoning/test_olmo3_reasoning_parser.py

Test output:

============================= test session starts ==============================
platform linux -- Python 3.12.11, pytest-8.4.2, pluggy-1.6.0
rootdir: /home/ec2-user/vllm
configfile: pyproject.toml
plugins: anyio-4.11.0, forked-1.6.0, cov-7.0.0, shard-0.1.2, asyncio-1.2.0, timeout-2.4.0, rerunfailures-16.0.1, mock-3.15.1, buildkite-test-collector-0.1.9, subtests-0.14.2, hypothesis-6.140.2, schemathesis-4.1.4, hydra-core-1.3.2, typeguard-4.4.4
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 14 items
Running 14 items in this shard

tests/reasoning/test_olmo3_reasoning_parser.py ..............            [100%]

============================== 14 passed in 3.50s ==============================

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the [Google Doc](https://docs.google.com/document/d/1YyVqrgX4gHTtrstbq8oWUImOyPCKSGnJ7xtTpmXzlRs/edit?tab=t.0).

[gemini-code-assist]

Code Review

This pull request introduces a reasoning parser for Olmo 3 models. The implementation for non-streaming mode appears correct, and the test suite is a good start. However, the streaming implementation has a critical correctness issue that can lead to data loss when a stream ends with a partial reasoning tag. I've also identified some apparently unreachable code in the buffer processing logic. My review includes detailed comments on these issues.

[DarkLight1337]

Can you open a follow-up PR to add it to https://docs.vllm.ai/en/latest/features/reasoning_outputs.html?