feat(flutter): derive OPEN_RUNTIMES_BUILD_KEYS from user variables

[jaysomani]

Captures user-supplied variable keys before executor merges its own internal vars, then adds OPEN_RUNTIMES_BUILD_KEYS as a space-separated list.

[greptile-apps]

Greptile Summary

The PR captures user-provided variable names before adding executor-managed variables and exposes those names through OPEN_RUNTIMES_BUILD_KEYS.

• Adds build-key propagation for runtime builds.
• Adds Node and Flutter end-to-end coverage for build-time variable injection.
• Makes the test client timeout configurable.
• Adjusts maintenance handling for runtimes under creation and container-removal failures.

Confidence Score: 5/5

The PR appears safe to merge because no blocking failure remains in the eligible follow-up review scope.

No blocking failure remains.

Important Files Changed

Filename	Overview
app/controllers.php	Captures user variable keys before internal-variable merging and passes them to build helpers through OPEN_RUNTIMES_BUILD_KEYS.
src/Executor/Runner/Maintenance.php	Excludes in-progress runtime creation from idle cleanup and prevents removal exceptions from escaping the maintenance batch.
tests/e2e/Client.php	Adds a configurable request timeout while preserving the existing 60-second default.
tests/e2e/ExecutorTest.php	Adds end-to-end tests for build-key propagation and Flutter compile-time variable injection.
tests/resources/functions/flutter/lib/main.dart	Adds a minimal Flutter fixture that renders a compile-time environment variable.
tests/resources/functions/flutter/pubspec.yaml	Defines the minimal Flutter test application and SDK constraints.
tests/resources/functions/flutter/web/index.html	Adds the Flutter web bootstrap page used by the end-to-end fixture.

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