perf: inline row decoding and eliminate closures in recv_results_rows

[mykaul]

Summary

• Split recv_results_rows into fast path (no column encryption) and slow path (CE enabled)
• Eliminate per-call closure allocation and merge two-pass row processing into single-pass decoding

Details

Problem

The current recv_results_rows has three sources of overhead on every call:

1. Two passes over row data: First recv_row reads all raw bytes into a list[list[bytes]], then decode_row iterates again to deserialize — doubling iteration and creating intermediate lists that are immediately discarded.

2. Per-call closures: decode_val and decode_row are defined as closures inside recv_results_rows, meaning Python allocates new function objects on every result set.

3. Unconditional ColDesc creation: ColDesc namedtuples are built for every column even when column encryption is not configured (the vast majority of deployments).

Solution

Fast path (no column encryption — the common case):

• _decode_row_inline(f, colcount, col_types, protocol_version) reads each column's size, reads the bytes, and immediately calls from_binary() — one pass, no intermediate list
• ColDesc creation is skipped entirely
• No closures allocated

Slow path (column encryption enabled):

• Preserves the existing two-pass logic (needed because CE must decrypt before type decoding)
• decode_val/decode_row moved to module-level functions (_decode_val_ce, _decode_row_ce) to avoid per-call closure overhead

Benchmark results

Scenario	Rows × Cols	Speedup
Standard decode	10 × 5	1.19×
Standard decode	100 × 5	1.12×
Standard decode	1000 × 5	1.12×
50% NULL columns	100 × 10	1.33×
50% NULL columns	1000 × 10	1.26×

The speedup is higher with NULL-heavy workloads because the inline path short-circuits from_binary() for negative-length (NULL) columns.

Merge conflict note

⚠️ This PR modifies the same recv_results_rows method as PR #630, which also splits the method into CE/non-CE branches. If both PRs are accepted, there will be a merge conflict requiring manual resolution.

Testing

• All 651 existing unit tests pass (16 pre-existing skips)
• No new tests added (this is a pure refactor of the decode path; the existing test_protocol.py and test_response_future.py tests exercise recv_results_rows via ResultMessage.recv_body)