fix: Prevent `_prepare_request_call` from mutating caller's headers dict #502

Job	Run time
PR title check	6s
Docstrings checks / Docstrings checks	14s
Code checks / Actions lint check	48s
Code checks / Spell check	8s
Tests / Integration tests (3.11, ubuntu-latest)	2m 20s
Tests / Integration tests (3.14, ubuntu-latest)	2m 20s
Doc checks / Doc checks / Docs check	1m 28s
Code checks / Type check / Type check (3.11)	13s
Code checks / Lint check / Lint check (3.14)	11s
Code checks / Type check / Type check (3.13)	14s
Code checks / Lint check / Lint check (3.11)	13s
Code checks / Type check / Type check (3.12)	15s
Code checks / Lint check / Lint check (3.12)	8s
Code checks / Type check / Type check (3.14)	12s
Code checks / Lint check / Lint check (3.13)	13s
Tests / Unit tests / Unit tests (windows-latest, 3.11)	1m 49s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.13)	38s
Tests / Unit tests / Unit tests (windows-latest, 3.12)	1m 18s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.12)	46s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.11)	37s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.14)	39s
Tests / Unit tests / Unit tests (windows-latest, 3.13)	1m 1s
Tests / Unit tests / Unit tests (windows-latest, 3.14)	54s
	16m 45s

Provide feedback