Fix SNI handling with proxies in requests library

[cosineai]

This pull request addresses issue psf#6900, which involves the inability to request a private URL endpoint with custom SNI and self-signed CA when a proxy is set. The changes made ensure that the server_hostname is correctly set based on the Host header in the request, even when using a proxy.

Changes Made:

• Modified the get_connection_with_tls_context method in adapters.py to extract the Host header and use it for SNI.
• Added unit tests in test_adapters.py to verify that the server_hostname is correctly passed in various scenarios, including when a proxy is used.

Expected Outcome:

With these changes, users should be able to successfully make requests to private URL endpoints with custom SNI and self-signed CA, regardless of whether a proxy is used.

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This pull request was co-created with Cosine Genie

Original Task: swe-requests/m99px2ucjsk6
Author: Pandelis Zembashis