RequestsHTTPTransport does not set headers on session like AIOHTTPTransport does

**Describe the bug**
When creating a `transport = RequestsHTTPTransport` with specific headers, `transport.session.headers` doesn't match what was given to the constructor (`and is in `transport.headers`)

This is at odd with the behaviour of `AIOHTTPTransport`, where we get `transport.session.headers == transport.headers`

**To Reproduce**
Steps to reproduce the behavior:

```
import pytest

from gql.transport.requests import RequestsHTTPTransport
from gql.transport.aiohttp import AIOHTTPTransport

@pytest.mark.asyncio
async def test_aio_http_transport_session():
transport = AIOHTTPTransport("url", headers={"test": "header"})
await transport.connect()
assert transport.headers == transport.session.headers

def test_requests_http_transport_session():
transport = RequestsHTTPTransport("url", headers={"test": "header"})
transport.connect()
assert transport.headers == transport.session.headers
```

```
$ uv run pytest -k test_gql -v 130 ↵ ✹ ✭ issue202/rest-client-headers
================================================= test session starts =================================================
platform linux -- Python 3.12.11, pytest-9.1.1, pluggy-1.6.0 -- /home/shtrom/work/simple-github/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /home/shtrom/work/simple-github
configfile: pyproject.toml
plugins: anyio-4.14.2, responses-0.6.0, aioresponses-0.3.0, mock-3.15.1, asyncio-1.4.0
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 34 items / 32 deselected / 2 selected

test/test_gql.py::test_aio_http_transport_session PASSED [ 50%]
test/test_gql.py::test_requests_http_transport_session FAILED [100%]

====================================================== FAILURES =======================================================
________________________________________ test_requests_http_transport_session _________________________________________

def test_requests_http_transport_session():
transport = RequestsHTTPTransport("url", headers={"test": "header"})
transport.connect()
> assert transport.headers == transport.session.headers
E AssertionError: assert {'test': 'header'} == {'User-Agent'... 'keep-alive'}
E
E Left contains 1 more item:
E {'test': 'header'}
E Right contains 4 more items:
E {'Accept': '*/*',
E 'Accept-Encoding': 'gzip, deflate, br, zstd',
E 'Connection': 'keep-alive',...
E
E ...Full output truncated (7 lines hidden), use '-vv' to show

test/test_gql.py:17: AssertionError
=============================================== short test summary info ===============================================
FAILED test/test_gql.py::test_requests_http_transport_session - AssertionError: assert {'test': 'header'} == {'User-Agent'... 'keep-alive'}
===================================== 1 failed, 1 passed, 32 deselected in 0.10s ======================================
```

**Expected behavior**
`transport.session.headers == transport.headers` for `RequestHTTPTransport`, same as `AIOHTTPTransport` already does.

**System info (please complete the following information):**
- OS: Linux x86_64, Ubuntu 26.04
- Python version: 3.14.4
- gql version: 4.0
- graphql-core version: 3.2.11

Comece com gql.transport.requests.RequestsHTTPTransport e a reprodução em test/test_gql.py, comparando seu comportamento de sessão com o de gql.transport.aiohttp.AIOHTTPTransport. Execute o teste direcionado para o transporte Requests e confirme que os cabeçalhos da sessão correspondem aos cabeçalhos do construtor, enquanto os testes de transporte existentes continuam passando.

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