[#39900] Fix MongoDbIO read splitting to preserve non-ObjectId _id types

[ogiogidayo]

Fixes #39900.

BoundedMongoDbSource.splitKeysToFilters formatted split boundaries unconditionally as ObjectId("..."), so collections keyed by non-ObjectId _id values (e.g. application-defined string ids, which MongoDB fully supports) failed with IllegalArgumentException: state should be: hexString has 24 characters as soon as read splitting was enabled via withBucketAuto(true) or withNumSplits(n). The AggregationQuery path (splitKeysToMatch) had the same assumption via getObjectId("_id").

This change serializes split boundaries with their actual BSON types:

• splitKeysToFilters now builds the range filters as Documents and serializes them with Document#toJson (extended JSON). ObjectId boundaries keep producing semantically identical filters, while string/integer ids are preserved.
• splitKeysToMatch keeps the raw _id value instead of reading it as an ObjectId. The generated $match documents for ObjectId ids are unchanged (the pre-existing expectations in testSplitIntoBucket pass without modification).

Backward compatible: no public API change. Note that testSplitIntoFilters previously asserted output that could never be parsed back (ObjectId("56") for an Integer id, which only passed because the test compared strings without parsing them); it now verifies the generated filters by parsing them. Added string-id tests for both paths.

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[ogiogidayo]

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[Abacn]

Thanks!