refactor (state management): Remove extra layer when mapping URL parameters to Explore state

[ericpgreen2]

In our current state mapping flow, we convert URLSearchParams → V1ExplorePreset → MetricsExplorerEntity. The intermediate V1ExplorePreset layer is unnecessary and adds complexity.

This PR removes the extra layer, directly mapping URLSearchParams to MetricsExplorerEntity.

Note: This PR doesn’t much cleanup the state mapping logic itself, but it simplifies the flow and sets us up for future cleanup.

Builds on #7084, #7087

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[AdityaHegde]

This sadly conflicts heavily with #7062. I am getting rid of the intermediate step in there already. This would be redundant.

[ericpgreen2]

This sadly conflicts heavily with #7062. I am getting rid of the intermediate step in there already. This would be redundant.

Nice to see that! Just this is the kind of thing that should be a standalone PR. Do you want to split your work out into a dedicated PR or should I get this one over-the-line?