feat(graph): add bulk graph import

[GITHUBTANMAY2003]

Description

Adds bulk graph import support for Python records, JSON files, and separate node/edge CSV files. The importer validates the complete input before writing, prepares each unique node once, and groups outgoing edges into bounded DynamoDB UpdateItem operations.

Related issue

Closes #35

Changes

• Added GraphStore.import_graph() and import_graph_file() with high-level Dynavec client wrappers.
• Added shared validation for Python, JSON, and CSV inputs with contextual record, filename, and row errors.
• Grouped edges by source and appended them in bounded chunks while preserving existing metadata, edges, document links, ordering, weights, properties, and namespace isolation.
• Preserved existing append semantics for duplicate edges and repeated imports.
• Added public documentation and comprehensive offline regression coverage.

Testing

• Tests pass locally
• Ruff checks pass
• Documentation updated, if applicable

Verification completed:

• Affected graph, client, namespace, and importer suite: 214 passed
• Async DynamoDB Moto test: 1 passed
• Semantic Kernel test in isolation: 8 passed
• Ruff: passed
• Formatting checks: passed
• Mypy: passed across 81 source files
• git diff --check: passed
• Full pytest suite passed with OpenAI pre-imported, with 2 skipped

The standard local make run-ci invocation completed Ruff and Mypy successfully but its pytest collection was blocked by an existing DSPy/OpenAI/Semantic Kernel circular-import ordering conflict. The complete pytest suite passes when openai is imported before pytest; no tests were excluded.

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

[Kaap10]

Verified and tested locally across all graph test suites; all 209 tests pass with clean validation, boundary handling, and zero regressions.

Thanks for your contribution @GITHUBTANMAY2003
LGTM. Ready to merge.