[KuroGeo]

What does this change?

The CLI currently passes every discovered package-local build.yaml as an explicit build file. If it configures only another builder, FlutterGen generates the requested assets and exits successfully but logs SEVERE: ... does not contain valid options. This also happens for workspace members.

Let the core configuration loader handle implicit discovery and look beside the selected pubspec. It only uses a discovered file when it contains FlutterGen options. Explicit --build inputs keep their existing diagnostics and precedence, and valid package-local options still work when --config points outside the working directory.

Related to #702. The current build_runner implementation already consumes BuilderOptions directly; I reproduced and fixed the remaining CLI path on current main.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Validation

On macOS arm64, Flutter 3.44.2 / Dart 3.12.2:

• dart run melos test: 106 passed across the command, core, runner and Flutter example suites. Unmodified baseline: 102 passed.
• dart run melos test:xml-matrix: 90 passed with xml 6.x and 90 with xml 7.x.
• dart run melos analyze: all six packages passed.
• Repository Dart format check and git diff --check: passed.
• Generated test goldens and examples through the CLI and build_runner, including workspace mode: no tracked output differences.

Before the production change, the two CLI regressions emitted SEVERE and the new package-local core discovery regression selected the wrong output directory. After the change they pass. The existing fallback test now writes its fixture beside the pubspec and verifies a freshly generated output path; it also fails on the original core source. A control verifies that an explicitly selected build file without options still reports SEVERE.

Windows/Linux CI and native application builds were not run locally. The full host test suite, including the existing build_runner workspace integration tests and Flutter widget tests, was run without adding skips or changing timeouts.

Checklist

• Existing issue identified before implementation (FlutterGen logs SEVERE when a build.yaml exists without FlutterGen options (config lives in pubspec.yaml) #702).
• Full melos test completed.
• Analyzer and formatter checks completed.
• Reviewed the final diff; no new public option or unrelated generated changes are included.
• Documentation update is not required: this preserves the existing CLI discovery behavior while removing the false diagnostic.

AI assistance: this patch and its tests were developed and reviewed with OpenAI Codex. The validation results above were run locally; upstream review and CI remain pending.