[LeonSGP (LeonSGP43)]

When the OCR plugin is registered without an OCR client, it currently replaces native PDF conversion with its own page.extract_text() pipeline. This contradicts the documented built-in fallback and loses table column positions and word spacing in machine-readable PDFs. For example, the existing sparse inventory fixture loses all 22 Markdown table lines, including the positions of blank cells.

Delegate to the existing PdfConverter when no OCR service is configured. Preserve the converter's stream rewind and forward conversion arguments. Configured OCR continues through its existing implementation. Regression tests exercise public plugin registration and conversion, plus direct conversion with an advanced stream cursor, using three existing real PDF fixtures.

Validation on macOS with Python 3.12.8 and sibling checkout imports:

• New public PDF regression cases: baseline 3 failed; fixed cases pass.
• Full OCR suite: 111 passed (16.01 seconds wall clock); comparable unmodified-main OCR suite: 108 passed (11.86 seconds).
• Full MCP suite: 21 passed (10.52 seconds); baseline: 21 passed (10.47 seconds).
• Full core suite: 1050 passed, 57 skipped (103.38 seconds). Executed python -m pytest in the already provisioned official Hatch test environment with GITHUB_ACTIONS=1; no new environment or dependency installation. The skips are the suite's conditional cases, not newly excluded tests.
• pre-commit run --all-files: Black passed. compileall, source import/runtime checks, and git diff --check passed.

Other Python/OS CI matrix legs and remote OCR services were not exercised locally. No model or external OCR call was required. This change restores the documented no-client fallback; it does not address configured-OCR scanning or layout issues in #2343 or #2566.

AI-assisted by gpt-6.1-sol; implementation and verification evidence were reviewed locally.

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