chore(transforms): migrate validate() to validate_structure()

[thomasqueirozb]

Summary

Completes the transform migration for RFC 2026-07-15-split-component-build-lifecycle by removing the deprecated validate() method from the TransformConfig trait and migrating all implementations to validate_structure().

Vector configuration

NA

How did you test this PR?

• cargo check passes
• cargo clippy passes
• cargo nextest run for all affected transforms (route, sample, throttle, reduce, exclusive_route, delay, tag_cardinality_limit)

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: chore(transforms): add validate_structure and rename validate_env #25953

[datadog-vectordotdev]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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