🔥 Remove betaEncodeCookieOptions and always encode cookie options

[thomas-lebeau]

⚠️ Stacked on #4157 — review that PR first

Motivation

The betaEncodeCookieOptions feature flag was introduced as a beta option to encode cookie options into the cookie value, mitigating micro-session issues caused by conflicting cookie configurations (e.g., when RUM and Logs SDKs are initialized with different trackSessionAcrossSubdomains or usePartitionedCrossSiteSessionCookie values).

This option has proven stable and is now ready to be promoted to the default behavior. Removing the beta flag simplifies the codebase and ensures all users benefit from encoded cookie options.

Changes

• Remove the betaEncodeCookieOptions init configuration option and its associated Configuration field
• Always encode cookie options into the session cookie value (previous behavior when betaEncodeCookieOptions: true)
• Simplify retrieveSessionCookie to always use the encoded cookie retrieval path, removing the configuration parameter
• Remove betaEncodeCookieOptions from serialized configuration and default context telemetry
• Update unit tests to reflect that cookie options are always encoded (cookie values now include &c=0 suffix)
• Simplify E2E tests for conflicting cookie options: remove the true/false loop and test.fail() guard since the behavior is now always correct

Test instructions

• Run unit tests: yarn test:unit --spec packages/core/src/domain/session/storeStrategies/sessionInCookie.spec.ts
• Run E2E session store tests: yarn test:e2e -g "Session Stores"
• Verify no regression in session handling with different cookie configurations

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	172.25 KiB	172.00 KiB	-259 B	-0.15%	✅
Rum Profiler	4.29 KiB	4.29 KiB	0 B	0.00%	✅
Rum Recorder	24.71 KiB	24.71 KiB	0 B	0.00%	✅
Logs	56.93 KiB	56.73 KiB	-206 B	-0.35%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	128.75 KiB	128.50 KiB	-259 B	-0.20%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.0064	0.0044	-31.25%
RUM - add action	0.0156	0.0154	-1.28%
RUM - add error	0.0154	0.0165	+7.14%
RUM - add timing	0.0042	0.0035	-16.67%
RUM - start view	0.0171	0.0154	-9.94%
RUM - start/stop session replay recording	0.0011	0.0008	-27.27%
Logs - log message	0.0202	0.0176	-12.87%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	26.35 KiB	27.31 KiB	+983 B
RUM - add action	49.85 KiB	51.40 KiB	+1.55 KiB
RUM - add timing	26.76 KiB	26.07 KiB	-714 B
RUM - add error	56.32 KiB	56.27 KiB	-52 B
RUM - start/stop session replay recording	26.05 KiB	25.40 KiB	-659 B
RUM - start view	451.29 KiB	453.38 KiB	+2.09 KiB
Logs - log message	45.77 KiB	46.26 KiB	+502 B

🔗 RealWorld

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 63.64%
• Overall Coverage: 77.09% (-0.05%)

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