refactor: resolve clippy warnings and apply idiomatic rust patterns

[marlon-costa-dc]

Summary

• Resolves various clippy warnings across the codebase.
• Replaces push_str(",") with push(',').
• Uses matches!() macro where appropriate.
• Adds is_empty() method to FieldColumn.
• Removes redundant manual Default implementations in favor of #[derive(Default)].
• Simplifies struct initializations (field init shorthand).

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[mergify]

@marlon-costa-dc Thanks for your contribution. Please submit with DCO, see the contributing guide https://github.com/milvus-io/milvus/blob/master/CONTRIBUTING.md#developer-certificate-of-origin-dco.

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