fix(core): [Session based Traces for Mobile 2] Keep transaction baggage mutable

[adinauer]

PR Stack (Session based Traces for Mobile)

• feat(core): Session based Traces for Mobile #5383
• feat(android): [Session based Traces for Mobile 1B] Add session trace lifecycle #5398
• fix(core): [Session based Traces for Mobile 2] Keep transaction baggage mutable #5402

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📜 Description

Keeps transaction baggage mutable when remapping a root transaction onto the current session trace. This lets each transaction populate its own DSC fields even if the ambient session propagation baggage was already frozen by an earlier outgoing request without an active transaction.

Adds regression coverage for:

• starting a new session
• producing outgoing headers without an active transaction, freezing the session baggage
• starting a transaction in the session trace and verifying outgoing transaction baggage
• finishing that transaction, then starting a second transaction and verifying its baggage too

💡 Motivation and Context

Session trace lifecycle can freeze ambient scope baggage before any transaction starts. If a later transaction reuses that frozen baggage directly, outgoing transaction propagation can miss transaction-specific DSC such as sentry-transaction, sentry-sampled, sentry-sample_rate, and sentry-sample_rand.

This keeps the session trace id and sample rand while allowing each remapped transaction to complete its own outgoing baggage.

💚 How did you test it?

• ./gradlew spotlessApply apiDump
• ./gradlew :sentry:test --tests 'io.sentry.BaggageTest' --tests 'io.sentry.ScopesTest.session trace transaction baggage is populated after scope baggage is frozen'

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

Continue the Session based Traces for Mobile stack.

#skip-changelog

⚠️ Merge this PR using a merge commit (not squash). Only the collection branch is squash-merged into main.

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.41.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	317.06 ms	375.78 ms	58.72 ms
Size	0 B	0 B	0 B

Baseline results on branch: ref/remove-propagation-context-mode

Startup times

Revision	Plain	With Sentry	Diff
f15a0c5	321.71 ms	365.46 ms	43.75 ms
c183fb7	368.02 ms	436.96 ms	68.94 ms

App size

Revision	Plain	With Sentry	Diff
f15a0c5	0 B	0 B	0 B
c183fb7	0 B	0 B	0 B