fix(arrow-avro): preserve object_store error source in the async reader

[ranflarion]

Which issue does this PR close?

• Closes arrow-avro: object_store error type is lost in the async reader (stringified into AvroError::General) #10493.

Rationale for this change

AvroObjectReader converts every error from the underlying ObjectStore with AvroError::General(e.to_string()), and the header fetch loop wraps its error the same way, so the typed object_store::Error is destroyed: source() returns nothing and no downcast is possible. Callers implementing retry or error-classification logic cannot distinguish a permanent NotFound from a transient transport fault without string matching. ParquetObjectReader already preserves the source via E: Into<ParquetError> in the identical spawn helper plus impl From<object_store::Error> for ParquetError; this aligns arrow-avro with that pattern.

What changes are included in this PR?

• impl From<object_store::Error> for AvroError (feature-gated on object_store), mapping to AvroError::External(Box::new(e)), which already reports its inner error via source() and maps through ArrowError::from_external_error.
• The private spawn helper bound changes from E: Error to E: Into<AvroError>, mirroring ParquetObjectReader::spawn.
• The header fetch loop propagates the reader error instead of reformatting it into AvroError::General.

Are these changes tested?

Two new tests in store.rs assert that a read of a missing object surfaces an error whose source() chain downcasts to object_store::Error::NotFound, with and without a spawn runtime. Existing suites pass unchanged.

Are there any user-facing changes?

Error Display text changes for store failures (External: ... instead of Avro error: ...). Variant selection within the #[non_exhaustive] AvroError is not a stability contract, so this is not considered a breaking API change.