fix(core): [Logs and Metrics Enable Flags 20] Make migration warnings visible

[adinauer]

PR Stack (Logs and Metrics Enable Flags)

• feat(core): Remove Logs and Metrics enable flags #5939
• feat(logback): [Logs and Metrics Enable Flags 1] Add Logs opt-in #5940
• feat(log4j2): [Logs and Metrics Enable Flags 2] Add Logs opt-in #5941
• feat(jul): [Logs and Metrics Enable Flags 3] Add Logs opt-in #5942
• feat(android): [Logs and Metrics Enable Flags 4] Add Timber Logs opt-in #5943
• feat(android): [Logs and Metrics Enable Flags 5] Add Logcat Logs opt-in #5945
• feat(spring): [Logs and Metrics Enable Flags 6] Add logging Logs opt-in #5946
• feat(core): [Logs and Metrics Enable Flags 7] Remove Logs enable flag #5947
• feat(android): [Logs and Metrics Enable Flags 8] Warn for legacy Logs metadata #5949
• feat(core): [Logs and Metrics Enable Flags 9] Warn for legacy Logs configuration #5950
• feat(spring): [Logs and Metrics Enable Flags 10] Warn for legacy Logs property #5951
• perf(core): [Logs and Metrics Enable Flags 11] Avoid unused Logs worker thread #5952
• feat(core): [Logs and Metrics Enable Flags 12] Remove Metrics enable flag #5953
• feat(android): [Logs and Metrics Enable Flags 13] Warn for legacy Metrics metadata #5954
• feat(core): [Logs and Metrics Enable Flags 14] Warn for legacy Metrics configuration #5955
• feat(spring): [Logs and Metrics Enable Flags 15] Warn for legacy Metrics property #5956
• perf(core): [Logs and Metrics Enable Flags 16] Avoid unused Metrics worker thread #5957
• fix(jul): [Logs and Metrics Enable Flags 17] Preserve null-message records #5960
• docs: [Logs and Metrics Enable Flags 18] Update options guidance #5961
• fix(android): [Logs and Metrics Enable Flags 19] Read Timber option lazily #5964
• fix(core): [Logs and Metrics Enable Flags 20] Make migration warnings visible #5966

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📜 Description

Routes obsolete Logs and Metrics enable-flag migration warnings through the always-visible fatal logger across core external configuration, Android manifest metadata, and all three Spring Boot variants.

Core now initializes its fatal logger before external configuration is merged. Android continues using AndroidFatalLogger, whose output is emitted at Log.ASSERT. Spring emits its warnings after Sentry.init, when the fatal logger is initialized.

💡 Motivation and Context

The migration warnings previously used the diagnostic logger. During external configuration merge that logger was still a no-op, and in Android or Spring normal startup it remained hidden unless SDK debug logging or a custom diagnostic logger was enabled.

These warnings are the migration path for removed aggregate options, so explicit obsolete configuration should be visible without asking users to enable debug logging.

💚 How did you test it?

• ./gradlew :sentry:test :sentry-android-core:testReleaseUnitTest :sentry-spring-boot:test :sentry-spring-boot-jakarta:test :sentry-spring-boot-4:test
• ./gradlew spotlessApply apiDump
• Verified JVM external configuration writes through SystemOutLogger with debug disabled
• Verified Android manifest warnings appear at Log.ASSERT with debug disabled
• Verified Spring warnings use a configured fatal logger without enabling debug logging

📝 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.

#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	8.52.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	393.18 ms	477.06 ms	83.88 ms
Size	0 B	0 B	0 B