Reopen test case for base64_url response body encoding

[Copilot]

Uncomments previously disabled test assertions for base64_url() response body encoding. These tests were temporarily commented out pending a fix in typespec-azure (#2411), which has now been released.

Changes

• Uncommented assert expected == client.response_body.base64_url() in test_encode_bytes.py
• Uncommented assert expected == await client.response_body.base64_url() in test_encode_bytes_async.py
• Added changelog entry

Reference: Azure/autorest.python#3253

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Original prompt

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This section details on the original issue you should resolve

<issue_title>[python] reopen test case</issue_title>
<issue_description>Reopen test case for https://github.com/microsoft/typespec/blob/5d549c414f9a359fd768f5130de4f029c8cb67f9/packages/http-client-python/generator/test/generic_mock_api_tests/test_encode_bytes.py and

typespec/packages/http-client-python/generator/test/generic_mock_api_tests/asynctests/test_encode_bytes_async.py

Lines 127 to 128 in 5d549c4

	# will reopen after TCGC release a fix version for https://github.com/Azure/typespec-azure/pull/2411
	# assert expected == await client.response_body.base64_url()

with reference of Azure/autorest.python#3253.

NOTE:

• DO add changelog</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [python] reopen test case #8838

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