GH-50231: Handle unset Substrait extension mapping type

[Reranko05]

Rationale for this change

A malformed Substrait SimpleExtensionDeclaration with an unset mapping_type currently reaches Unreachable(), causing the process to abort during deserialization. Since this can be triggered by malformed input, the deserializer should instead return an Invalid status.

What changes are included in this PR?

• Replace the Unreachable() call in GetExtensionSetFromMessage with a Status::Invalid return for unset or unknown mapping_type values.
• Add a regression test to verify that deserializing a Substrait plan containing an empty SimpleExtensionDeclaration returns an Invalid status instead of aborting.

Are these changes tested?

Yes. A regression test covering the malformed SimpleExtensionDeclaration case has been added to serde_test.cc.

Are there any user-facing changes?

No.

This PR contains a "Critical Fix".

This change prevents process termination when deserializing malformed Substrait plans or expressions by returning a proper error instead of calling std::abort().

• GitHub Issue: [C++] Apache Arrow Substrait deserialization reaches Unreachable() / std::abort() on a SimpleExtensionDeclaration with an unset mapping_type oneof #50231

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.