feat: add Azure Linux ecosystem

[JamieMagee]

Overview

Adds Azure Linux as a queryable ecosystem so the API can match pkg:rpm/azure-linux/ PURLs to AZL-* vulnerability records.

Ref: #5041

Details

Azure Linux is an RPM-based distro, so this reuses the existing RPM ecosystem helper (no new version-comparison logic needed).

Changes:

• _ecosystems.py: registered 'Azure Linux': RPM
• purl_helpers.py: added PURL mapping pkg:rpm/azure-linux/<package>
• ecosystem.go: added osvconstants.EcosystemAzureLinux -> rpmEcosystem

Ecosystem name (Azure Linux) and PURL namespace (azure-linux) match the published vulnerability data at https://github.com/microsoft/AzureLinuxVulnerabilityData and the schema PR ossf/osv-schema#498.

Testing

• Added forward and reverse PURL conversion tests in purl_helpers_test.py
• Added 'Azure Linux' to the RPM ecosystem list in redhat_test.py
• Added a Go version-parsing test case in wrapper_test.go (TestZeroVersion)

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[another-rex]

LGTM, though we might need to wait a few days to get this merged in.

[another-rex]

Can you have a look at the CLA check as well? Thanks!

[another-rex]

/gcbrun

[JamieMagee]

Can you have a look at the CLA check as well? Thanks!

I forgot to hit rescan. Looks good now!

[another-rex]

Thanks! The CI failures is due to some issues on our end, unrelated to the changes in this PR, we'll get that fixed!