Increase default JSON string buffer capacity

[sb123sb123]

Fixes #2645

Why

The implicit JSON string output paths still started with the default StringBuilder capacity of 16. For a 33-character string, the JSON output is 35 characters but the builder grows to a capacity of 70, causing an avoidable resize and copy. This is the remaining issue discussed after #2852 switched the implicit output path from StringWriter to StringBuilder.

What changed

• Centralize the initial capacity for Gson-created JSON string builders at 256 characters.
• Use that capacity for Gson.toJson(...), JsonElement.toString(), and TypeAdapter.toJson(...).
• Leave caller-provided Appendable instances unchanged.

Tests

• Focused GsonTest: 15 tests, 0 failures.
• Gson module test suite: 4,674 tests, 0 failures, 21 skipped.
• mvn -B -ntp -pl gson -Dmaven.repo.local= verify: passed, including compilation, ProGuard processing, OSGi integration tests, Spotless checks, and packaging.

The full multi-module mvn clean verify was not run because the shared Windows G: drive was concurrently space-constrained; the affected gson module completed verify.

AI assistance

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[eamonnmcmanus]

Thanks for taking this on! It does need a small adjustment.

[eamonnmcmanus]

Unfortunately this doesn't work because we need to be able to build a subset of Gson that includes JsonElement but does not include Gson. That's why the build for this PR is failing. You could move the newJsonStringBuilder() method here as a quick fix. That's not a great home for it, but I don't see a better one without introducing a new class.