✨ [PANA-6624] Enable the new session replay serialization algorithm by default

[sethfowler-datadog]

Motivation

This PR makes the new session replay serialization algorithm the default in V7.

Changes

• The experimental feature flags have been removed.
• The internal API functions now return records in the new format. (I've confirmed that all callers can handle this.)
• The recording code no longer directly depends on the old serialization code, allowing it to be excluded from the bundle. (Some tests do still depend on it, so the code can't be removed yet, but we'll get there.)
• Some session replay tests had two variants: one version tested the old format, and one version tested the new one. In cases where the variant testing the old format no longer works (e.g. the E2E tests, which relied on the removed experimental feature flags), that variant has been removed.
• Many other session replay tests have been updated.

Test instructions

It should be enough to test recording with yarn dev in the usual way.

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

[sethfowler-datadog]

Effects on bundle size for rum_recorder:

Format	Status Quo	This PR	Improvement
Uncompressed	27.10 KiB	21.27 KiB	21.5%
Compressed	9.27 KiB	7.73 KiB	16.6%

[gonzalezreal]

🚀

[datadog-datadog-prod-us1-2]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 77.61% (+0.14%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 5fe92ee | Docs | Datadog PR Page | Give us feedback!}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	168.88 KiB	168.71 KiB	-169 B	-0.10%	✅
Rum Profiler	6.09 KiB	6.09 KiB	0 B	0.00%	✅
Rum Recorder	27.10 KiB	21.25 KiB	-5.85 KiB	-21.60%	✅
Logs	54.47 KiB	54.36 KiB	-108 B	-0.19%	✅
Rum Slim	127.17 KiB	127.07 KiB	-108 B	-0.08%	✅
Worker	22.99 KiB	22.99 KiB	0 B	0.00%	✅

🚀 CPU Performance

Pending...

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	38.40 KiB	39.84 KiB	+1.44 KiB
RUM - add action	64.63 KiB	62.65 KiB	-1.98 KiB
RUM - add timing	36.61 KiB	43.05 KiB	+6.43 KiB
RUM - add error	70.23 KiB	70.01 KiB	-226 B
RUM - start/stop session replay recording	41.46 KiB	41.04 KiB	-425 B
RUM - start view	475.32 KiB	475.56 KiB	+239 B
Logs - log message	54.53 KiB	59.69 KiB	+5.16 KiB

🔗 RealWorld