xberg E2E Fixtures

One JSON object per file under fixtures/<dir>/. The directory is the default category; an explicit category field overrides it and names the generated test file (e.g. fixtures/plugin_api/list_reranker_backends.json sets reranker_backend_management, so its Python test lands in e2e/python/tests/test_reranker_backend_management.py).

• Fields: id (unique snake_case, becomes the test name), call, description, docs (topic, title, description, side_effects), assertions; optional category, tags, input, config, args, skip.
• call: extract (most fixtures), extract_batch, pdf_page_count, extract_with_external_redaction, and the plugin-registry calls (list_*, register_*, unregister_*, clear_* for OCR, embedding, tokenizer, reranker backends, renderers, validators, post-processors; ocr_backend_supports_language).
• input: extract_input (kind: "uri" with $mock_url/..., or bytes + filename + mime_type) plus mock_responses (path, status_code, headers, optional method, and a body: body_file resolved against a responses subdirectory of fixtures/ (none exists yet), then fixtures/ itself, so ../test_documents/... is the in-repo submodule; or body_inline). Batch calls take inputs. A bytes input can also sit at the top level of input (kind: "bytes", bytes, filename, mime_type) without extract_input. Other calls take call-specific input fields (pdf_bytes/password, findings_json, registry name/backend); copy an existing fixture of that call.
• Assertion types: not_error, error, equals, min_length, count_min, count_equals, contains, contains_any, contains_all, not_contains, not_empty, greater_than_or_equal, less_than_or_equal. Example field: results[0].mime_type.
• Assertion shape and generation: the shared e2e-generation skill.
• skip: {languages: [...], reason: "..."}.
• Directories: smoke, extract, batch, contract, error, format_specific, pdf, code, url, plugin_api, registry, summarization. Field-only categories (all under plugin_api/): ocr_backend_management, embedding_backend_management, tokenizer_backend_management, reranker_backend_management, renderer_management, validator_management, post_processor_management.
• fixtures/repros/ holds repro assets (PDFs, configs, READMEs) and fixtures/images/ a test image; neither holds e2e fixtures.

Copy the nearest existing fixture of the same call rather than writing one from this list.