Check custom type edge case

@voonhous LGTM but can you check this one weird case in case a user would try this (unlikley but sharing below):

```

The validator code

(descriptorType, f.dataType) match {
case (HoodieSchemaType.BLOB, st: StructType) => validateBlobStructure(st)
case (HoodieSchemaType.VARIANT, st: StructType) => validateVariantStructure(st)
case _ => // <-- silently no-op
}

The pattern only matches when both the tag says BLOB and the data type is a
StructType. Anything else falls into case _ and does nothing.

The bug, concretely

Suppose a user (or a buggy upstream transform) builds this schema:

val blobMetadata = new MetadataBuilder()
.putString(HoodieSchema.TYPE_METADATA_FIELD, HoodieSchemaType.BLOB.name())
.build()

val schema = new StructType()
.add("id", LongType)
.add("payload", LongType, nullable = true, metadata = blobMetadata)
// ^^^^^^^^ ^^^^^^^^^^^^
// wrong type says "I'm a BLOB"

The user is asserting "payload is a BLOB" via the metadata, but the data type is a
LongType, not the canonical BLOB struct.

What happens today

1. validateCustomTypeStructures(schema) runs.
2. It sees the hudi_type=BLOB tag on payload.
3. The match tuple is (BLOB, LongType) — neither pattern matches → falls into case _
→ returns without throwing.
4. Then convertStructTypeToHoodieSchema runs.
5. The BLOB case in toHoodieTypeNested is case blobStruct: StructType if
metadata.contains(...) && ...isCanonicalBlobStruct(blobStruct) => — requires a
StructType, so it doesn't match either.
6. LongType falls through to the normal case LongType => HoodieSchema.create(LONG)
arm.
7. Result: the field is silently written as a plain LONG. The BLOB tag is ignored,
no error.

The user thinks they wrote a BLOB column; the table actually has a LONG column.

The fix

Add an explicit reject for "tag says BLOB/VARIANT but the type is wrong":

(descriptorType, f.dataType) match {
case (HoodieSchemaType.BLOB, st: StructType) => validateBlobStructure(st)
case (HoodieSchemaType.VARIANT, st: StructType) => validateVariantStructure(st)
case (HoodieSchemaType.BLOB, other) =>
throw new IllegalArgumentException(
s"Field '${f.name}' is tagged hudi_type=BLOB but has type $other; expected a
StructType.")
case (HoodieSchemaType.VARIANT, other) =>
throw new IllegalArgumentException(
s"Field '${f.name}' is tagged hudi_type=VARIANT but has type $other; expected
a StructType.")
case _ =>
}

Now the misuse fails fast at the write boundary instead of silently producing the
wrong on-disk schema.
```

_Originally posted by @rahil-c in https://github.com/apache/hudi/issues/18566#issuecomment-4322589008_

Search for validateCustomTypeStructures, validateBlobStructure, validateVariantStructure, and toHoodieTypeNested to locate the validator and conversion path. Reproduce the tagged LongType example, then run the relevant schema tests if present; done means mismatched BLOB or VARIANT metadata fails instead of being converted as a plain type.

Written by the indexing model from the issue text.