fix: Faulty parsing of customized image's tag_set

[jopemachine]

Fixed incorrect tag_set parsing of customized images generated via the session commit introduced in PR #1909.

Before & After

For example, in the case of the following image, the tag_set data of the parsing result is updated as shown below.

Image canonical: registry.gitlab.com/project/img_name:3.9-ubuntu20.04-customized_ecab90f3618b4ad394c737200c7f5a44

Before

{'ubuntu': '20.04', 'customized_ecab': '90f3618b4ad394c737200c7f5a44'}

After

{'ubuntu': '20.04', 'bai_customized_image_tag': 'customized_ecab90f3618b4ad394c737200c7f5a44'}

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Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation

[jopemachine]

Let’s avoid embedding special keywords or gimmicks directly into the parsing logic.
Instead of adding special keywords like bai_customized_image_tag, let’s consider a better approach.

[HyeockJinKim]

Shouldn't you remove customized_ from tag?

[jopemachine]

In this PR, I mappped the tag itself to the special keyword bai_customized_image_tag.
If we’re only mapping the UUID of the custom image, it might be better to rename the keyword to something like bai_customized_image_id.