manage_build blocks on unsaved scene save dialog

[TeapoyY]

Problem

When manage_build triggers a player build while the open scene has unsaved changes, Unity shows a modal Save modified scenes? dialog. Because MCP runs headlessly/automated, the build hangs until a user dismisses the dialog manually.

Expected behavior

manage_build should save assets and open scenes automatically before calling BuildPipeline.BuildPlayer, so automated builds are not blocked by save prompts.

Steps to reproduce

1. Open a scene in the Unity Editor and make a change without saving.
2. Call manage_build with action=build.
3. Observe Unity prompts to save modified scenes and the build does not proceed until the dialog is handled.

Proposed fix

Call AssetDatabase.SaveAssets() and EditorSceneManager.SaveOpenScenes() in BuildRunner immediately before BuildPipeline.BuildPlayer runs (same pattern already used by TestRunnerService for test runs).

Environment

• Unity MCP manage_build tool
• Any Unity version where BuildPipeline.BuildPlayer prompts for unsaved scenes

[TeapoyY]

Fix submitted in #1340.