GH-33823: [C++] Improve err msgs when opening files that are the wrong format

[RobertLD]

Rationale for this change

The original error messages did not provide instruction to users on how to best correct their usage

What changes are included in this PR?

Improving the messages

Are these changes tested?

Current unit tests were executed, no new tests were introduced

Are there any user-facing changes?

Yes, error messages have been updated that are surfaced to users

• GitHub Issue: [C++] Improve error message when reading Streaming file with File reader and vice versa #33823

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[Copilot]

Pull request overview

This PR improves user-facing error messages in the C++ IPC readers/decoders when callers attempt to open an IPC File as a Stream (or vice versa), providing more actionable guidance aligned with GH-33823.

Changes:

• Update the IPC file reader footer/magic check failure message to suggest the streaming reader when appropriate.
• Add a heuristic in the IPC stream message decoder to detect an IPC file magic prefix being misinterpreted as a stream message length, and emit a more instructive error.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cpp/src/arrow/ipc/reader.cc	Improves the “not an Arrow file” error when the footer magic doesn’t match, suggesting the streaming reader.
cpp/src/arrow/ipc/message.cc	Adds detection for IPC File magic bytes when decoding a stream message and returns a more targeted “wrong format” error.

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[Copilot]

The error message suggests calling open_stream(), but this is not a C++ API entry point (C++ uses RecordBatchStreamReader::Open / stream reader types). Since this Status message is emitted from the C++ library and may be surfaced to C++ callers as well as bindings, consider making the guidance API-neutral (e.g., refer to the IPC stream reader) or mention the C++ API name explicitly.

Suggested change

	"Not an Arrow file. If this is an Arrow IPC Streaming format file, try "
	"open_stream() instead.");
	"Not an Arrow file. If this is an Arrow IPC streaming format file, use "
	"the IPC stream reader instead.");

[Copilot]

This error text is emitted from the C++ IPC decoder but recommends open_file() / open_stream(), which are binding-specific names (they don't exist in the C++ API). Consider API-neutral guidance (IPC file reader vs IPC stream reader) or include the C++ entry points (e.g., RecordBatchFileReader::Open / RecordBatchStreamReader::Open). Also, "Arrow IPC File format file" repeats "file"; reword to avoid the redundancy.

Suggested change

	". This appears to be an Arrow IPC File format file. "
	"Try open_file() instead of open_stream().");
	". This appears to be an Arrow IPC file. "
	"Try the IPC file reader instead of the IPC stream reader.");

[RobertLD]

Decent feedback Mr. Bot, I'll make changes shortly. Honestly impressed the LLM review had such good context. Good bot