[DOCS-13406] Add guide for conditional logic in Synthetic tests

[git-thuerk-done]

What does this PR do? What is the motivation?

This stems from many support tickets and feature requests from customers requesting more information on conditional branching.

Fixes DOCS-13406
Adds a new guide covering how to implement conditional logic in Synthetic tests using subtests and step execution options. The guide addresses a documentation gap around the existing step execution options (allowFailure, exitIfSucceed,
alwaysExecute), which are available in the product but hard to discover and understand.

The guide covers three patterns for Browser, Multistep API, and Mobile App tests:

• If/else-style flows — run different test paths based on which UI state the application presents (for example, A/B testing scenarios)
• Element state-based logic — interact with the first available element using a custom locator and allow-failure behavior
• Always-run cleanup steps — execute teardown steps regardless of whether earlier steps succeeded

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/alicia/docs-13406-conditional-logic-synthetic-tests/synthetics/guide/conditional-logic-subtests

Modified Files

• https://docs-staging.datadoghq.com/alicia/docs-13406-conditional-logic-synthetic-tests/synthetics/guide/

[domalessi]

Left some comments and suggestions! Hit me up if you have Qs, and let me know when this is ready for another look :)

[domalessi]

Does the "always run this step" apply even if we have "If this step succeeds, stop this test and mark it as passed" earlier up in the chain?

[git-thuerk-done]

Yes for this we want to capture the error and continue directly to step X from the step where it failed (i.e. in this pattern we don't want the "If this step succeeds, stop this test and mark it as passed" condition). hope that makes sense!