chore(config): Handle serde(untagged) in enum variants

[bruceg]

Summary

serde allows the last variant in an internally tagged enum to be marked as untagged. This allows it to handle a schema that does not use the tag attribute for one of the variants. This change adds support to the configurable macro to accomodate this capability.

Vector configuration

N/A

How did you test this PR?

A new smoke test is included which passes.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[Copilot]

Pull Request Overview

This PR adds support for handling serde(untagged) in enum variants within the configurable macro. It allows the last variant in an internally tagged enum to be marked as untagged, which enables handling schemas that don't use the tag attribute for specific variants.

Key changes:

• Added detection and validation of untagged variants in tagged enums
• Implemented proper schema generation for untagged variants
• Added comprehensive test coverage for the new functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
lib/vector-config/tests/integration/smoke.rs	Adds integration test for tagged enum with trailing untagged variant schema generation
lib/vector-config-macros/src/ast/variant.rs	Modifies variant parsing to detect and handle serde(untagged) attributes
lib/vector-config-macros/src/ast/util.rs	Adds utility function to check for flag-style attributes and refactors existing attribute parsing
lib/vector-config-macros/src/ast/container.rs	Adds validation logic to ensure untagged variants are positioned correctly and used appropriately

[tessneau]

lg from my end