[client][rust] Align fixed-schema scans by column ID

[slfan1989]

Purpose

Linked issue: close #3770

The Rust fixed-schema log scanner previously aligned historical Arrow batches with the scanner schema by field name.

This behavior was inconsistent with the KV FixedSchemaDecoder, which uses stable Fluss column IDs. Name-based alignment may lose values when a column keeps the same ID but changes its name, or incorrectly reuse values when
different column IDs have the same name.

This PR aligns fixed-schema log reads by column ID, consistent with FixedSchemaDecoder.

Brief change log

• Capture the fixed target Fluss schema when creating a fixed-schema log scanner.
• Build source-to-target mappings with the existing index_mapping utility, using stable column IDs.
• Pass the mapping to Arrow ReadContext for both local and remote log reads.
• Preserve values when source and target columns have the same ID.
• Fill target columns with NULL when their IDs do not exist in the source schema.
• Prevent columns with the same name but different IDs from being incorrectly matched.
• Add regression tests for local and remote fixed-schema decoding.

Tests

Added Rust unit tests.

API and Format

No public API or storage format changes.

Documentation

No documentation changes are required.

[slfan1989]

While reviewing the Rust Arrow fixed-schema read path, I noticed that historical data is currently aligned by field name rather than by stable column ID. Under typical schema-evolution semantics, this could prevent old data from being matched correctly after a column rename, so I initially considered it a bug.

However, after local testing and further investigation, I confirmed that Fluss currently supports only a limited set of schema-evolution operations, such as adding nullable columns. Although the client exposes a RENAME COLUMN interface, the corresponding functionality has not yet been implemented on the server. Therefore, this issue cannot currently be reproduced through the complete client-server workflow.

Nevertheless, the Arrow fixed-schema path is inconsistent with the KV FixedSchemaDecoder in how fields are aligned. To provide consistent semantics across both read paths and establish a reliable foundation for supporting additional schema-evolution operations in the future, I submit a PR that changes Arrow fixed-schema alignment from field names to stable column IDs.