[client] Return exception for partitioned tables in Rust client

[Arnav-panjla]

Purpose

Linked issue: close #99

The current Rust client does not handle partitioned tables. To prevent incorrect usage,
this PR adds validation to return an UnsupportedOperation error when
new_append() or new_scan() is called on a partitioned table.

Brief change log

• Added new UnsupportedOperation error variant in crates/fluss/src/error.rs to handle unsupported operations
• Added partitioned table validation in FlussTable::new_append() - returns UnsupportedOperation error if the table is partitioned
• Changed FlussTable::new_scan() return type from TableScan to Result<TableScan> and added partitioned table validation
• Updated all call sites to handle the new Result return type:
  • crates/examples/src/example_table.rs
  • crates/fluss/tests/integration/table.rs
  • crates/fluss/tests/integration/table_remote_scan.rs
  • bindings/python/src/table.rs
  • bindings/cpp/src/lib.rs
• Updated doc examples in crates/fluss/src/client/table/scanner.rs

Tests

• All existing unit tests pass (cargo test --lib)
• Integration tests updated to handle the new return type
• Verified with cargo check, cargo clippy, and cargo fmt

API and Format

API Change: Yes

• FlussTable::new_scan() return type changed from TableScan<'_> to Result<TableScan<'_>>
• This is a breaking change for callers of new_scan(), but necessary to properly communicate the error condition

Storage Format: No

Documentation

No.