Set `linker` object in `gtag` within the Ads tag

## Feature Description

[Unlike Analytics](https://github.com/google/site-kit-wp/blob/0db6c72b707829d5bf2808db358acc91da30f6a7/includes/Modules/Analytics_4/Web_Tag.php#L141-L145), the Ads module does not set a linker object in `gtag` within its tag. This should be addressed in such a way so that when both Analytics and Ads modules are connected, only one `linker` object is set.

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## Acceptance criteria

*

## Implementation Brief

* [ ]

### Test Coverage

*

## QA Brief

*

## Changelog entry

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Start by comparing the Ads tag with includes/Modules/Analytics_4/Web_Tag.php, especially the referenced gtag linker setup. Trace how both module tags are emitted and verify that the Ads tag sets the linker object without producing duplicate linker objects when Analytics and Ads are connected.

Written by the indexing model from the issue text.