(improvement) row_parser: cache ParseDesc for prepared statements

Cache the ParseDesc object constructed in recv_results_rows() so that
repeated executions of the same prepared statement skip the list
comprehensions, ColDesc construction, and make_deserializers() call.

The cache is keyed by id(column_metadata). For prepared statements the
result_metadata list is stored on PreparedStatement and reused, so id()
is stable. On cache hit we verify object identity (cached_ref is
column_metadata) and that session-level settings (column_encryption_policy,
protocol_version) still match.

Implementation details:
- _get_or_build_parse_desc: cached path, used only when column_metadata
  comes from result_metadata (prepared statements with stable id()).
- _build_parse_desc: uncached path, used for inline metadata from
  non-prepared queries that creates a fresh list every execution.
- Cache is bounded to 256 entries; cleared entirely when full.
- Returns only (desc, column_names, column_types) to the caller,
  avoiding exposure of internal cache fields.
- Thread-safety: dict get/set are atomic in CPython (GIL), but
  concurrent cache misses may cause redundant construction (benign).

A clear_parse_desc_cache() and get_parse_desc_cache_size() function
are exposed for testing.

## Benchmark results (median, pytest-benchmark)

### ParseDesc construction only (reference benchmarks)

| Columns | **Before** (original) | **After** (with cache) |
|---------|-----------------------|------------------------|
| 5 cols  | 3,966 ns              | 191 ns                 |
| 10 cols | 5,730 ns              | 175 ns                 |
| 20 cols | 9,266 ns              | 166 ns                 |
| 50 cols | 19,388 ns             | 193 ns                 |

### Full pipeline integration (recv_results_rows through Cython)

| Scenario          | **Before** (original) | **After** (with cache) |
|-------------------|-----------------------|------------------------|
| 1 row x 10 col    | 40,867 ns             | 2,977 ns               |
| 100 rows x 5 col  | 145,584 ns            | 73,206 ns              |
| 1000 rows x 5 col | 1,099,825 ns          | 999,517 ns             |

For small result sets (single-row lookups common with prepared statements),
ParseDesc construction is a large fraction of the total response-path cost.
Caching eliminates it entirely after the first execution.

All 623 unit tests pass (16 skipped - pre-existing).

Author: mykaul