[pigeon] Deprecate dartHostTestHandler

[stuartmorgan-g]

Deprecates dartHostTestHandler, in preparation for removing sometime in 2026 if no significant new information comes to light.

Part of flutter/flutter#178322

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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[stuartmorgan-g]

test-exempt: deprecating existing code

[gemini-code-assist]

Code Review

This pull request effectively deprecates dartHostTestHandler and its associated option dartTestOut. The changes are consistent across the codebase, including updates to the API, options, version, and changelog.

To make the deprecation more complete, you might also consider deprecating the dart_test_out command-line option in _argParser (in packages/pigeon/lib/src/pigeon_lib.dart) by marking it as hidden (hide: true) and updating its help text. This would make the command-line interface consistent with the API deprecation.

I have one minor suggestion to make the changelog entry more explicit.