Add v2 page-memory schema support

[suguanYang]

Summary

• add api_version support with default v1 behavior and per-call overrides for jobs, documents, polling/loading, retrieval, and high-level parse calls
• add v2 page-memory models and parsing: page chunks, content_source, page summaries, metadata.entities, page_chunks, retrieval chunk_types, and expanded data_type
• update docs, README examples, and tests for v2 endpoints and page chunks

Notes

• The API-side schema work is in Add v2 page-memory ingestion API knowhere#196.

Verification

• uv run ruff check .
• uv run mypy src tests
• uv run pytest -q (248 passed)
• uv build
• git diff --check

[suguanYang]

Follow-up update for the v2-only direction:

• Removed the public api_version / v1 parse-flow switching surface.
• Python SDK requests now always use /v2 for parse, jobs, documents, polling, and retrieval.
• Version metadata is now 2.0.0 (pyproject.toml, src/knowhere/_version.py, release manifest/changelog).
• Legacy parsed-result chunk inspection remains for existing v1 result ZIPs.

Validation:

• uv run ruff check .
• uv run mypy src tests
• uv run pytest -q (245 passed)
• uv build
• git diff --check

Remote CI is green for Python 3.9 through 3.13 on commit cfa4ac9.