fix: normalize upgrade changelog URL

[Bortlesboat]

Summary

• normalize stale CortexLM/cortex-cli changelog URLs to CortexLM/cortex before converting GitHub URLs to raw content
• add an integration regression that serves a release with the stale URL and verifies cortex upgrade --changelog --check fetches the corrected changelog path

Related to PlatformNetwork/bounty-challenge#51509.

Tests

• cargo test -p cortex-cli --test upgrade_changelog_url stale_cortex_cli_changelog_url_uses_cortex_repo -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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