♻️ use global URL constructor and fall back to pristine iframe only when needed

[BenoitZugmeyer]

Motivation

The SDK creates a hidden <iframe> on every page just to read an unpatched URL constructor, even though the global URL works correctly in the vast majority of environments. This is wasteful and can interfere with strict sandboxed contexts (e.g. Shopify Custom Pixels). We should only fall back to the iframe when the global constructor is actually patched or broken.

Changes

• buildUrl now probes the global URL constructor with a relative-URL resolution check and uses it directly when it behaves correctly, falling back to the pristine iframe-sourced constructor only when needed.
• Removed the HTMLIFrameElement.prototype.contentWindow shim from the Shopify sandbox patch, no longer necessary now that the iframe path is not taken by default.
• Moved urlPolyfill.spec.ts next to its source file in js-core.

Test instructions

• In the sandbox, load a page and confirm view URLs resolve correctly (no hidden iframe created in the DOM).
• Temporarily override window.URL with a throwing function in the console, then trigger a navigation: URLs should still resolve via the iframe fallback.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 85.71%
• Overall Coverage: 77.23% (+0.12%)

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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	181.20 KiB	181.31 KiB	+113 B	+0.06%	✅
Rum Profiler	8.43 KiB	8.43 KiB	0 B	0.00%	✅
Rum Recorder	21.12 KiB	21.12 KiB	0 B	0.00%	✅
Logs	57.52 KiB	57.63 KiB	+111 B	+0.19%	✅
Rum Salesforce	N/A	139.33 KiB	N/A	N/A	N/A
Rum Slim	139.21 KiB	139.32 KiB	+112 B	+0.08%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	200.92 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.43 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	3.72 KiB	N/A	N/A	N/A

[chatgpt-codex-connector]

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