[py] Use ssl_proxy descriptor when initializing Proxy from raw dict - #18237

Description

When initializing a Proxy instance from a dictionary, all proxy properties (http_proxy, o_proxy, proxy_autoconfig_url, auto_detect, socks_proxy, etc.) use their corresponding descriptor on Proxy, except sslProxy which directly assigned self.sslProxy = raw["sslProxy"].

Setting self.sslProxy directly bypassed the _ProxyTypeDescriptor, causing two issues when sslProxy is supplied without httpProxy:

1. proxy.proxy_type remained ProxyType.UNSPECIFIED instead of transitioning to ProxyType.MANUAL.
2. As a result, o_capabilities() emitted {'proxyType': 'unspecified', ...} and o_bidi_dict() omitted sslProxy completely.
3. Incompatible proxy type verification (_verify_proxy_type_compatibility) was skipped.

This PR changes self.sslProxy = raw["sslProxy"] to self.ssl_proxy = raw["sslProxy"] and adds tests to verify proxy.proxy_type is correctly set to MANUAL and compatibility is enforced.

Motivation and Context

Fixes unexpected UNSPECIFIED proxy type and missing proxy configuration when creating a Proxy from a dictionary containing sslProxy.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.