Restore project description from private metadata when .project is missing

[Copilot]

When a project's .project file is deleted but the workspace metadata contains natures and buildspec (added in eclipse-platform#2033), opening the project now succeeds by restoring from metadata instead of failing with an error dialog.

Changes

FileSystemResourceManager.java - Modified read() to attempt recovery:

• When .project is missing, create description from private metadata
• If natures or buildspec present, log warning and return restored description
• If no useful data, throw original error (backward compatible)
• Restored description is written to disk by existing SaveManager.saveMetaInfo() flow

IProjectTest.java - Added test coverage:

• testOpenProjectWithMissingDotProjectButPrivateMetadata verifies project opens successfully and .project is recreated after workspace save

Behavior

Before:

Project closed → Delete .project → Try to open → Error: "project description file missing"

After:

Project closed → Delete .project → Try to open → Success (if metadata has natures/buildspec)
Warning logged: "project description file was missing... A new file has been created"
Next workspace save → .project recreated

Fixes eclipse-platform#2272

Original prompt

Propose a fix for eclipse-platform#2272

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