[Python][C++] Fix tree-model TsFileDataFrame returning empty data on reused readers and uppercase names (#862)

* fix: correct tree-model TsFileDataFrame reads on reused readers and uppercase names

Two related fixes for reading tree-model TsFiles through TsFileDataFrame, which together caused 'read one series OK, then reading another series returns empty' on a reused reader.

C++: make StringArrayDeviceID::split_table_name()/init_prefix_segments() idempotent. Device IDs are cached and reused across queries; the previous code appended to prefix_segments_ on every call, so prefixes accumulated (and leaked) across successive queries and corrupted per-device path columns, making a second series read match nothing. Now it clears/frees existing prefixes before rebuilding. Adds DeviceIdTest.SplitTableNameIsIdempotent.

Python: read series with uppercase measurement/path names. query_table_on_tree synthesizes a case-insensitive table schema, so the native layer lower-cases column names and path segments, while the dataset catalog keeps the original casing. The tree read paths matched field columns and device paths case-sensitively and returned empty for uppercase names. _read_series_by_row_tree and _read_arrow_tree now match case-insensitively. Adds a regression test.

* style(python): apply spotless (black) formatting to tree-model regression test

* revert(python): drop case-insensitive tree-model matching; add case-collision regression test

Revert the .lower() case-insensitive column/path matching in the tree read paths (from d13cd236). It let single-variant uppercase measurements read, but conflated case-distinct measurements (e.g. temperature vs Temperature) into one series. Restore case-sensitive matching (native still lower-cases names, so uppercase reads are empty for now) and add a regression test asserting case-distinct measurements do not collide. Two tree-model tests are intentionally red pending the C++ root fix (TableSchema for tree queries must not lower-case names).

* fix(cpp): preserve case for tree-derived (virtual) table schemas

Tree-model device paths and measurement names are case-sensitive, but the

virtual TableSchema synthesized in query_on_tree lower-cased them via the

TableSchema constructor. This collapsed case-distinct series (e.g.

"temperature" vs "Temperature") into one column, so reads returned empty

or mixed data.

- TableSchema: add virtual_table flag; skip lower-casing table/measurement

  names and keep column_pos_index_ in original case for virtual tables.

- find_column_index/find_id_column_order: match case-sensitively for

  virtual tables, unchanged (lower-cased) for real tables.

- query_on_tree: build the synthesized schema with virtual_table=true.

* docs(cpp): trim case-sensitivity comments in TableSchema

* spotless