Fix: improve Java exception error messages with cause chain (legacy #250)

[HenryNebula]

Summary

Fixes #80: Java exceptions from JDBC drivers now produce clean, informative error messages that include the full cause chain.

Previously, when a JDBC driver threw a non-SQLException (e.g., RuntimeException), the error message was uninformative — users would see generic "Java Exception" without context about the root cause. Additionally, JPype 1.7.0+ introduced duplicated class-name prefixes (e.g., java.lang.java.lang.RuntimeException).

Changes

• Add _build_java_exception_message() to extract clean class name, message, and walk the getCause() chain for informative error output
• Avoid JPype 1.7.0+ duplicated class-name artefact by using getClass().getName() instead of str(exc)
• Wrap prepareStatement() and executeBatch() in try/except so Java exceptions from these calls are properly handled as DB-API errors (previously they propagated raw)
• Add mockExceptionOnExecuteWithCause() to MockDriver for testing cause chains
• Add mock tests for cause chain inclusion and no duplicate classnames
• Add integration test verifying SQL exceptions produce clean messages with cause chain

Test plan

• Added mock test reproducing RuntimeException with nested cause
• Added mock test verifying no duplicate class names in error messages
• Added integration test verifying SQL exceptions include cause chain
• Verified fix passes with CLASSPATH="test/jars/*:test/mock-jars/*" uv run python -m unittest test.test_mock (62 tests)
• Verified fix passes with CLASSPATH="test/jars/*" uv run python -m unittest test.test_integration.HsqldbTest (17 tests)

Closes #80

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