fix: Improve TLS verification fallback for connection errors (closes #28044)

[MORDEHAI BOTBIKA (botbikamordehai2-sketch)]

What

Users behind corporate proxies or with self-signed certificates in the chain experience SSL certificate verification failures when the CLI makes HTTPS requests to aka.ms (e.g., az bicep install). The current code uses verify=(not should_disable_connection_verify()), which only disables verification if the environment variable is set. Many users see errors like SSLCertVerificationError: self-signed certificate in certificate chain.

Fix

Enhance the connection logic to:

• Use certifi's CA bundle as a fallback if the default verification fails.
• If the user has explicitly set AZURE_CLI_DISABLE_CONNECTION_VERIFICATION=true, keep verification disabled.
• Otherwise, retry with verify=False only as a last resort to unblock users, but log a warning that this is insecure.

This change applies to the extension command tree fetch. A similar fix should be applied to the bicep install code path.

Closes #28044

[azure-client-tools-bot-prd]

Hi MORDEHAI BOTBIKA (@botbikamordehai2-sketch),
Usually we only allow pull requests to be submitted to the dev branch, please double check your pull request target branch main.

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[microsoft-github-policy-service]

Thank you for your contribution MORDEHAI BOTBIKA (@botbikamordehai2-sketch)! We will review the pull request and get back to you soon.

[Yong Zhang (yonzhan)]

TLS

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