GH-10613: Fix IPC FileReader projected schema

[dhruvxvaishnav]

Which issue does this PR close?

• Closes ipc file reader RecordBatchReader::schema ignores column projection #10613.

Rationale for this change

When FileReader uses a projection, the batches contain the projected schema but RecordBatchReader::schema() returns the complete file schema. This makes the schema reported by the reader inconsistent with the batches it produces.

What changes are included in this PR?

• Store the projected output schema on FileReader while keeping the complete schema in FileDecoder for decoding.
• Return the projected schema from FileReader::schema().
• Validate projection indices when constructing the reader.
• Add regression tests for projected schemas and invalid projections.

Are these changes tested?

Yes. I ran:

• cargo test -p arrow-ipc --all-features
• cargo test -p arrow --all-features
• cargo clippy -p arrow-ipc --all-targets --all-features -- -D warnings
• cargo +stable fmt --all -- --check

Are there any user-facing changes?

Yes. Projected IPC file readers now report the same schema as the record batches they produce. There are no public API signature changes.

AI assistance: OpenAI Codex assisted with codebase exploration, implementation support, test preparation, and validation. I reviewed and validated the changes and remain responsible for understanding and maintaining them.

[Jefffrey]

thanks @dhruvxvaishnav