Sanitize raw exception messages in indexing and import diagnostics

[Widthdom]

Found while dogfooding the locally built cdidx on branch codex/search-audit-issues-20260619.

Duplicate check: no open issue found for sanitize raw exception messages indexing import manifest diagnostics ex.Message.

Evidence:

• src/CodeIndex/Cli/IndexCommandRunner.FullScan.cs:27 falls back to raw ex.Message for index file exceptions.
• src/CodeIndex/Cli/ExportImportCommandRunner.cs:658 returns InvalidDataException.Message from manifest reads.
• Similar diagnostics already use CommandErrorWriter.FormatSanitizedException in other paths.

Problem:
Raw exception messages can include user-controlled paths or payload fragments. Index/import diagnostics should use consistent bounded and redacted formatting.

Acceptance criteria:

• Replace raw exception-message fallbacks in indexing/import diagnostics with sanitized bounded formatting.
• Preserve specific regex timeout and structured manifest depth messages.
• Add tests for long path/payload fragments in exception messages.

[Widthdom]

Work started on branch fix-issue3660-3796-3793-3740-3724-3806-3804.