♻️ refactor modifiable field paths in rum assembly

[BenoitZugmeyer]

Motivation

simplify maintenance of beforeSend modifiable fields and reduce bundle size a bit.

Changes

• factor shared modifiable paths into one common constant
• define per-event modifiable paths in a single static map
• remove unnecessary RumEventType.VIEW_UPDATE variant

Test instructions

1. Start the sandbox with yarn dev.
2. Set beforeSend to modify allowed fields (for example view.url or error.message) and verify sent events are updated.
3. Try modifying a non-allowed field and verify it is not applied.
4. Return false in beforeSend for a non-view event (dropped) and for a view event (not dropped).

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

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	175.47 KiB	175.36 KiB	-113 B	-0.06%	✅
Rum Profiler	8.08 KiB	8.08 KiB	0 B	0.00%	✅
Rum Recorder	21.23 KiB	21.23 KiB	0 B	0.00%	✅
Logs	57.01 KiB	57.01 KiB	0 B	0.00%	✅
Rum Slim	133.08 KiB	132.97 KiB	-114 B	-0.08%	✅
Worker	22.99 KiB	22.99 KiB	0 B	0.00%	✅

🔗 RealWorld

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 76.56% (-0.01%)

_{This comment will be updated automatically if new data arrives.
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