Add support for 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 ==============================

---

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.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request adds a reasoning parser for Olmo 3 models. The implementation includes a new parser class, Olmo3ReasoningParser, which handles both streaming and non-streaming outputs by operating on strings to find <think> and </think> tags. Unit tests are also included to verify the parser's behavior. The overall approach is sound, but I've found a critical issue in the string overlap detection logic that could lead to incorrect parsing under certain tokenization scenarios. The rest of the changes, including the test cases and module exports, look good.

[gemini-code-assist]

The loops for detecting string overlaps miss single-character overlaps. The range function's stop parameter is exclusive. Using 1 as the stop value in range(len(a) - 1, 1, -1) means the loop will not execute for an overlap of length 1.

For example, if checking for an overlap between "...a" and "b..." where the overlap is a single character, the current implementation would fail. This could cause the parser to miss <think> or </think> tags if they are split across tokens in a way that results in a single-character overlap.

To fix this, the stop value in the range calls should be 0 to include checks for overlaps of length 1.

Suggested change

	for i in range(len(a) - 1, 1, -1):
	if a[-i:] == b[:i]:
	ind_a = Indices(len(a) - i, len(a))
	ind_b = Indices(0, i)
	return (ind_b, ind_a) if swap else (ind_a, ind_b)
	# third check: does the beginning of a overlap with
	# the end of b?
	for i in range(len(a) - 1, 1, -1):
	if b[-i:] == a[:i]:
	ind_a = Indices(0, i)
	ind_b = Indices(len(b) - i, len(b))
	return (ind_b, ind_a) if swap else (ind_a, ind_b)
	for i in range(len(a) - 1, 0, -1):
	if a[-i:] == b[:i]:
	ind_a = Indices(len(a) - i, len(a))
	ind_b = Indices(0, i)
	return (ind_b, ind_a) if swap else (ind_a, ind_b)
	# third check: does the beginning of a overlap with
	# the end of b?
	for i in range(len(a) - 1, 0, -1):
	if b[-i:] == a[:i]:
	ind_a = Indices(0, i)
	ind_b = Indices(len(b) - i, len(b))
	return (ind_b, ind_a) if swap else (ind_a, ind_b)

[soldni]

Will open a new one with DSO.