No validation that --db path is a valid CodeIndex database before executing query

[Widthdom]

Summary

WithDb() checks File.Exists for non-URI paths and catches exceptions, but doesn't validate DB schema version, integrity, or compatibility. A file.db that exists but is corrupted, empty, or from an incompatible cdidx version can reach DbContext.TryMigrateForRead() which logs errors but continues, potentially returning incorrect/partial results silently. CLI doesn't exit with a clear error if the DB is not a valid CodeIndex database.

Where

• src/CodeIndex/Cli/QueryCommandRunner.cs:3117 (File.Exists check)
• src/CodeIndex/Cli/QueryCommandRunner.cs:3127 (DbContext constructor no validation)
• src/CodeIndex/Cli/QueryCommandRunner.cs:3128 (TryMigrateForRead swallows errors)

Suggested approach

1. Add a DbContext.TryValidateIsCodeIndexDb() method that checks for required tables (codeindex_meta, symbols, references)
2. Call this after DbContext construction but before DbReader creation in WithDb()
3. If validation fails, print a specific error: "Error: does not appear to be a valid CodeIndex database"
4. Include a hint: "Rebuild with cdidx index <projectPath> --db <path> to create a fresh database"
5. Return CommandExitCodes.DatabaseError when validation fails
6. Write tests for corrupted/truncated/wrong-schema DBs

[Widthdom]

Work started on branch fix-issue2025-2026-1988-2037.