Optimize response formatter application hot path

[yabirgb]

Avoid allocating a nested helper function on every _apply_response_formatters call and replace repeated formatter membership checks plus indexed lookups with single dict.get() lookups.

This reduces overhead on the common no-formatter path while preserving the existing non-mutating response behavior, result formatting, subscription result formatting, and error formatting.

codex wrote this benchmark https://gist.github.com/yabirgb/e29c583a71f52069522f6fb2d326dd03

that gives

~/Documents/web3.py clean-deps !1 ?3 > uv run python benchs/benchmark_apply_response_formatters.py --number 10000 --repeat 2
warning: No `requires-python` value found in the workspace. Defaulting to `>=3.13`.
number=10000 repeat=2
case           legacy_best_s  current_best_s  speedup  legacy_mean_s  current_mean_s
----------------------------------------------------------------------------------
no_formatter          0.0030          0.0022     1.38x         0.0032          0.0024
result                0.0054          0.0041     1.32x         0.0057          0.0041
subscription          0.0073          0.0067     1.09x         0.0074          0.0074
error                 0.0082          0.0056     1.46x         0.0083          0.0059