Add optional scan-on-open for Rust files

[dollfins]

Description

Currently, the extension only scans on save (onDidSaveTextDocument). It does not scan files when they are first opened. A user opening a Soroban contract for the first time must either save it or manually run the scan command to see diagnostics.

Current Behavior

File is scanned only on save:

vscode.workspace.onDidSaveTextDocument((doc) => {
    if (doc.languageId === 'rust' && runOnSave) {
        diagnosticsProvider.scanFile(doc);
    }
})

Expected Behavior

Add an optional Scan on Open feature:

1. When a Rust file is opened and it has not been scanned yet in this session, automatically scan it
2. Add a configuration setting sorobanGuard.runOnOpen (default: true)
3. Track which files have been scanned in a Set<string> to avoid redundant scans
4. Reset the tracking set when the extension is deactivated or when Clear Results is run

Implementation Notes

• Register vscode.window.onDidChangeActiveTextEditor to detect when a new file is opened
• Use a Set<string> of scanned file paths to avoid re-scanning
• Respect the existing exclude patterns
• Show a brief "scanning on open" status in the status bar
• Ensure this does not cause excessive scanning when switching between open tabs

Acceptance Criteria

• Rust files are automatically scanned when first opened
• New setting sorobanGuard.runOnOpen controls this behavior
• Already-scanned files are not re-scanned on tab switch
• Exclude patterns are respected
• No performance impact on editor startup

Complexity

Medium — requires understanding of editor lifecycle events and performance considerations.

Points

150