perf: replace functools.partial with closure in _query callback (100-200ns improvement - 1.3-1.6x speedup)

[mykaul]

Motivation

ResponseFuture._query() creates a functools.partial(self._set_result, host, connection, pool) on every single query (line 4623). This is the primary response callback, executed for every request. partial has non-trivial allocation overhead: it creates a new partial C object, allocates a tuple for positional args, and invocation goes through partial.__call__ indirection.

Summary

• Replace partial(self._set_result, host, connection, pool) with a closure using default-argument capture
• The closure avoids the partial object allocation and __call__ indirection
• Default-argument capture stores the bound values directly in the function's __defaults__ tuple, making both creation and invocation cheaper

Before

cb = partial(self._set_result, host, connection, pool)

After

_set_result = self._set_result
def cb(response, _h=host, _c=connection, _p=pool, _sr=_set_result):
    _sr(_h, _c, _p, response)

Testing

• All 30 unit tests in tests/unit/test_cluster.py pass
• The callback signature is preserved: cb(response) calls self._set_result(host, connection, pool, response) identically
• Other partial uses in cluster.py (connection factory, watchers, reprepare, timeout rescheduling) are left unchanged as they are on cold/rare paths

Benchmarks

Isolated benchmark measuring callback creation + 1 invocation (5M iterations):

Pattern	Creation	Invocation	Total	Speedup
functools.partial (before)	343.9 ns	91.8 ns	435.7 ns/call	—
Closure (default-arg) (after)	192.0 ns	76.3 ns	268.3 ns/call	1.62x
Closure (simple)	260.4 ns	69.8 ns	330.3 ns/call	1.32x

The default-argument closure was chosen as it has the best total cost (creation + invocation), since both happen exactly once per request.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.

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[Copilot]

Pull request overview

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This PR optimizes the hot-path response callback in ResponseFuture._query() by replacing a per-request functools.partial(...) allocation with a closure that captures the required arguments via default parameters.

Changes:

• Replace partial(self._set_result, host, connection, pool) with a closure capturing host/connection/pool and _set_result.
• Reduce callback creation + invocation overhead on every request (per PR benchmarks).

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