feat: support reading dictionary-encoded columns over Arrow Flight

[owencorrigan]

What's Changed

The C# Arrow Flight reader (RecordBatchReaderImplementation) threw
NotImplementedException on any dictionary batch, so a Flight stream containing a
dictionary-encoded column could not be read.

• ReadNextRecordBatchAsync now loops over the incoming Flight messages, feeding
  each through the base CreateArrowObjectFromMessage (which reads dictionary
  batches into the DictionaryMemo and returns null for them) until it yields a
  record batch, instead of throwing on any non-record-batch message. Handles both
  single and replacement dictionaries.
• The schema is decoded using the reader's DictionaryMemo via a new base helper
  ReadSchemaFromMessage, so dictionary-encoded fields are registered
  (FlightMessageSerializer previously discarded them).

The change routes dictionary batches to the existing ArrowReaderImplementation
decoder, so it is not specific to any particular index or value type. Validated
against a pyarrow RecordBatchStream serving a dictionary<int16, string> column
(single + replacement dictionaries); the current reader throws on the same stream.

Refs #180 (receive side).

Generated-By: Claude Opus 4.8