chore(transforms): add validate_structure and rename validate_env

[thomasqueirozb]

Summary

Implements steps 1 and 2 of the transform migration from RFC 2026-07-15. Adds validate_structure() for context-free validation and renames validate_env() to validate_with_context() for context-dependent validation.

Vector configuration

NA

How did you test this PR?

• make check-clippy passes
• Unit tests for affected transforms pass

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

• Related: docs(rfc): separate structural validation from component build #25848

[datadog-vectordotdev]

 

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Changelog | validate-changelog     

ℹ️ Info

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🧪 All tests passed
❄️ No new flaky tests detected

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