fix: accept fenced JSON output in raw JSON mode for OpenAI-compatible backends

[Samay10]

Description

Fixes issue #414 where vLLM/lmdeploy OpenAI-compatible backends may return fenced JSON output even when LangExtract is configured for raw JSON mode. This updates langextract/core/format_handler.py so FormatHandler._extract_content() detects and strips a single fenced JSON block when use_fences=False, and adds a regression test for fenced JSON output in raw JSON mode.

Fixes #414

Bug fix

How Has This Been Tested?

• Added regression test in tests/format_handler_test.py
• Verified modified files compile with python3 -m py_compile
• Full unit tests not run in the current environment due to missing absl dependency

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed Google's Python Style Guide and ran pylint over the affected code.

[github-actions]

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