json_schema_builder: Prepare v0.1.6 release.

[ditman]

Description

Moving publishable features out of a "wip" changelog version. See guidelines here.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).
• If my PR is a fork PR, I've checked that [e2e tests] passed.

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[gemini-code-assist]

Code Review

This pull request updates the CHANGELOG.md for json_schema_builder to finalize the release of version 0.1.6. There are no review comments, and I have no feedback to provide.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[github-actions]

Package publishing

If you have publishing permissions, you can use the links below to publish the changes after merging this PR.

Package	Version	Status	Publish tag (post-merge)
package:a2ui_core	0.0.1-wip002	ready to publish	a2ui_core-v0.0.1-wip002
package:genai_primitives	0.2.4	ready to publish	genai_primitives-v0.2.4
package:genui	0.10.0	ready to publish	genui-v0.10.0
package:genui_a2a	0.10.0	ready to publish	genui_a2a-v0.10.0
package:json_schema_builder	0.1.6	ready to publish	json_schema_builder-v0.1.6

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.