[config] Add validation for numeric config fields

[Prajwal-banakar]

Purpose

Linked issue: close #385

Add a validate_numeric_fields() method to Config that validates all numeric fields have sensible values (e.g. sizes > 0, timeouts >= 0). Validation is called during FlussConnection::new() alongside the existing validate_security() and validate_scanner_fetch() checks.

Brief change log

• Added validate_numeric_fields() in config.rs covering 10 numeric fields
• Called validate_numeric_fields() in FlussConnection::new() in connection.rs
• Added 11 unit tests covering default validity and each invalid case

Tests

cargo fmt --all
cargo clippy -p fluss-rs --all-features
cargo test -p fluss-rs

API and Format

API: No breaking changes
Storage format: No

Documentation

No doc changes needed.