[rust] Return TableNotExist for missing tables

[adamrecsko]

Generative AI disclosure

• No generative AI tools used
• Yes - OpenAI Codex

Purpose

Linked issue: close #4270

FlussConnection::get_table currently returns InvalidTableException when the requested table does not exist. This prevents callers from reliably distinguishing a
missing table from network, timeout, or other RPC failures.

This change makes get_table return FlussError::TableNotExist when the table is absent while preserving other errors unchanged.

Brief change log

• Check whether the requested table exists before refreshing its metadata.
• Return a typed TableNotExist error when the table is absent.
• Preserve network and other RPC errors without converting them.
• Add an integration test covering the error classification.

Tests

The following checks were run:

• cargo fmt --all -- --check
• RUST_TEST_THREADS=1 RUST_LOG=DEBUG RUST_BACKTRACE=full cargo test --features integration_tests --test test_fluss -p fluss-rs -- --nocapture

The new integration test passed:

• test_get_nonexistent_table_returns_table_not_exist

Full integration-suite result:

• 77 tests passed.
• 5 filter-pushdown tests failed
• These failures happen during statistics-enabled table creation and are unrelated to the get_table error-classification change.

API and Format

This change does not modify the public API, wire protocol, or storage format. It corrects the error code returned by the existing FlussConnection::get_table API.