build(deps): update openai-agents requirement to >=0.12,<0.18

[study8677]

Summary

• Manual replay of Dependabot PR build(deps): update openai-agents requirement from <0.15,>=0.12 to >=0.12,<0.18 in /engine #74 (openai-agents <0.15 -> <0.18).
• Dependabot's auto-rebase conflicted with fix(scanner): support Python 3.10 by falling back to tomli #77 (scanner 3.10 fix) because both PRs touched engine/pyproject.toml.
• This PR carries the same single-line dependency bump on top of fresh main.
• Supersedes build(deps): update openai-agents requirement from <0.15,>=0.12 to >=0.12,<0.18 in /engine #74.

Test plan

• CI green on all 8 checks (3.10/3.11/3.12 engine + cli, contract, docker).