perf(core): [SDK Overhead reduction for JVM 3] Reduce writer buffer size

[adinauer]

PR Stack (SDK Overhead reduction for JVM)

• perf(core): SDK Overhead reduction for JVM #5535
• perf(core): [SDK Overhead reduction for JVM 1] Remove redundant event map copies #5536
• perf(core): [SDK Overhead reduction for JVM 2] Short-circuit combined scope breadcrumbs #5541
• perf(core): [SDK Overhead reduction for JVM 3] Reduce writer buffer size #5544

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📜 Description

Set an explicit 512-character BufferedWriter buffer for envelope item and envelope serialization.

This keeps the existing OutputStreamWriter-based UTF-8 encoding path and does not introduce a custom writer implementation.

💡 Motivation and Context

This implements the safe AR-13 serialization buffer-size optimization from the SDK overhead reduction research. The JDK BufferedWriter default allocates an 8192-character buffer for each short-lived serialization writer, which is oversized for typical envelope item JSON payloads.

Using a smaller explicit buffer reduces temporary allocation overhead while preserving serialization behavior.

💚 How did you test it?

• ./gradlew spotlessApply apiDump
• ./gradlew :sentry:test --tests io.sentry.SentryEnvelopeItemTest --tests io.sentry.JsonSerializerTest

📝 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

#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	8.44.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	317.56 ms	364.46 ms	46.90 ms
Size	0 B	0 B	0 B

Baseline results on branch: perf/sdk-overhead-jvm-v2

Startup times

Revision	Plain	With Sentry	Diff
d346c71	324.53 ms	363.54 ms	39.01 ms

App size

Revision	Plain	With Sentry	Diff
d346c71	0 B	0 B	0 B

Previous results on branch: perf/sdk-overhead-jvm-writer-buffer-512

Startup times

Revision	Plain	With Sentry	Diff
226835a	314.28 ms	367.69 ms	53.41 ms
f7243b2	320.19 ms	374.33 ms	54.14 ms

App size

Revision	Plain	With Sentry	Diff
226835a	0 B	0 B	0 B
f7243b2	0 B	0 B	0 B