fix: improve error handling for environment variable parsing

[XuQianJin-Stars]

Replace unwrap() calls with proper error handling in environment variable parsing functions to prevent panics when invalid values are provided.

Changes:

• get_default_batch_size(): Handle invalid LANCE_DEFAULT_BATCH_SIZE values
• DEFAULT_FRAGMENT_READAHEAD: Handle invalid LANCE_DEFAULT_FRAGMENT_READAHEAD values
• DEFAULT_XTR_OVERFETCH: Handle invalid LANCE_XTR_OVERFETCH values
• DEFAULT_IO_BUFFER_SIZE: Handle invalid LANCE_DEFAULT_IO_BUFFER_SIZE values

When parsing fails, the functions now:

1. Log a warning message with the invalid value and error
2. Fall back to default values gracefully
3. Prevent application crashes due to configuration errors

Added test_env_var_parsing() to verify the new error handling behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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