Fix proxy host overrides in destination configs

[cookesan]

Summary

• Allows destination config files to replace proxy.host with proxy.hosts, or the reverse, without leaving both keys in the merged config.
• Preserves the validation error for configs that set both keys in the same proxy config.
• Adds regression coverage for both destination override directions.

Fixes #1832.

Testing

• bundle _2.6.5_ exec ruby -Itest test/configuration/proxy_test.rb in a ruby:3.4 container
• bundle _2.6.5_ exec ruby -Itest test/configuration/validation_test.rb in a ruby:3.4 container
• git diff --check

[Copilot]

Pull request overview

This PR fixes a config-merge edge case where destination-specific config files can switch between proxy.host and proxy.hosts (in either direction) without leaving both keys present in the merged proxy config, while keeping the existing validation behavior for configs that explicitly set both keys.

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Changes:

• Adjust config file loading to remove the superseded proxy.host/proxy.hosts key from the already-loaded config when a later file overrides with the alternate key.
• Add regression tests covering both override directions (host → hosts and hosts → host) via destination config files.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/kamal/configuration.rb	Removes replaced proxy.host/proxy.hosts keys during destination config merges to avoid invalid merged configs.
test/configuration/proxy_test.rb	Adds regression coverage for destination overrides switching between host and hosts.

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[cookesan]

Superseded by #1903, which covers the same destination override behavior and adds the role-level cases.