fix: reject aggregation on sequence fields

[QuakeWang]

Purpose

Linked issue: #257

Rust aggregation schema validation allowed fields.<sequence-field>.aggregate-function, but the runtime ignores that option because sequence fields are forced to last_value. Java rejects this metadata during schema validation, so Rust could create table options that are not accepted by Java validation.

Brief change log

• Reject per-field aggregate functions on sequence.field columns during aggregation schema validation.
• Keep runtime sequence-field merging unchanged: sequence fields still use last_value for existing or external metadata.
• Add unit and DataFusion SQL coverage for create/alter validation and ignored primary-key functions.
• Document the sequence-field aggregation restriction.

Tests

• cargo fmt --all -- --check
• cargo clippy --all-targets --workspace --features fulltext,vortex,mosaic -- -D warnings
• cargo test -p paimon --lib aggregation
• cargo test -p paimon --lib schema
• cargo test -p paimon-datafusion --test pk_tables aggregation

API and Format

No storage format change. This tightens validation for invalid aggregation table options.

Documentation

Updated SQL docs for sequence-field aggregation behavior.