Tell the model how many rows a Parquet or Avro file holds

[kaxil]

Follow-up to #73899. ObjectStorageToolset's read_file returned a Parquet or Avro file as its schema and first 20 rows, with no row count, and silently ignored offset and limit. In a run against a real model, asked how many rows a Parquet file held, the agent called read_file again with offset=21, got the same 20 rows back, and answered that it could not tell.

The result now opens with the row count and says that offset and limit do not apply:

Rows: 250. The schema and the first 20 rows follow; offset and limit do not apply to Parquet files.
Schema: order_id: int64, region: string, total: double
Sample rows:
...

The header comes before the sample, so cutting a long result never drops it. Parquet's count comes from the file footer. An Avro file is read in one pass over its blocks: the sample comes from the first blocks and the count from every block header, so only the blocks the sample reaches have their records decoded, and the file is opened once, under the existing max_read_bytes check. LLMFileAnalysisOperator keeps its current Avro path. A file whose Avro schema is not a record now shows its values in the sample instead of an empty list.

Counting reaches every block, which exposed a gap: a corrupt deflate block raises zlib.error, and corrupt .xz text raises lzma.LZMAError, neither of which is an OSError or ValueError, so either one failed the run. Both are now refused like any other unreadable file, and the model is told why.

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[Lee-W]

Suggested change

	f"offset and limit do not apply to {columnar.capitalize()} files.\n"
	)
	return _cut(header + sample.text, self._max_output_bytes)
	f"offset and limit do not apply to {columnar.capitalize()} files."
	)
	return _cut(f"{header}\n{sample.text}", self._max_output_bytes)

non-blocking, but i prefer doing something like this. although the difference is not noticeable