feat: improve environment-based client setup

[xxvw]

Summary

• add configurable D1Client.builderFromEnv() and D1AsyncClient.builderFromEnv() entry points
• preserve zero, sub-millisecond, and oversized timeout semantics in the default transport
• make the quickstart version overridable and validate examples in CI on Java 17
• expand Maven, Gradle Kotlin DSL, PowerShell, timeout, and environment-builder documentation
• consolidate the Unreleased notes for PRs fix: make nested response values immutable #111, feat: expose raw batch failure details #112, fix: make async client close graceful #114, and this change

Related Issue

Closes #110

Type of Change

• Feature
• Bug fix
• Test
• Documentation
• Refactor
• Build / CI
• Release

Checklist

• I created a branch before making changes.
• I did not commit directly to main.
• I used this pull request template.
• I linked the relevant issue where possible.
• I added or updated tests.
• I updated documentation if needed.
• I updated CHANGELOG.md.
• I ran mvn clean verify.
• I did not include names of automated development tools in project artifacts.
• I did not expose API tokens, secrets, private keys, account IDs, database IDs, customer data, or sensitive data.
• I did not add new runtime dependencies without an issue and maintainer approval.

Testing

mvn -B clean verify javadoc:javadoc
mvn -B -DskipTests install
mvn -B -f examples/quickstart/pom.xml -Dcloudflare-d1-java.version=0.2.0 compile
mvn -B -f examples/quickstart/pom.xml -Dcloudflare-d1-java.version=0.2.0 exec:java -Dexec.mainClass=example.CustomTransportExample
actionlint .github/workflows/ci.yml

All 110 tests passed. The credential-free custom transport example printed {value=1}.

Notes

No runtime dependencies were added. The new public APIs use Java 8-compatible types. Merge this pull request after #111, #112, and #114 so the consolidated Unreleased notes describe changes already present on main.