feat(bendpy): improve local embedded python api

[sundy-li]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

• improve bendpy local embedded runtime stability by switching embedded listeners to ephemeral ports
• add a Python-side local API facade with connect(), relation helpers, and dataframe registration helpers
• add regression coverage for the DuckDB-style local API behavior

Implementation

1. Configure embedded MySQL/HTTP/Flight/Admin/Metric listeners to bind 127.0.0.1 with port 0
2. Export the Rust classes needed by the Python facade from the PyO3 module
3. Add a Python wrapper layer in src/bendpy/python/databend/__init__.py for connect, SessionContext, Relation, register, and from_df
4. Enable maturin python-source packaging so the Python facade ships in the wheel
5. Add tests covering query execution, fetch helpers, and dataframe/Arrow registration

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Pair with the reviewer to explain why

Validation used locally:

• built bendpy wheel with Python 3.12
• exercised real local embedded queries through the built wheel
• verified bendsql Python local integration against the rebuilt wheel

Type of change

• New feature (non-breaking change which adds functionality)

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This change is 

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🤖 CI Job Analysis

Workflow: 24282318350

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• Total Jobs: 87
• Failed Jobs: 1
• Retryable: 0
• Code Issues: 1

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