Return structured JSON for bounded cursor validation errors

[Widthdom]

Summary

Bounded find cursor failures previously wrote human diagnostics to stderr even when JSON was explicitly selected. The shared bounded-response validator now emits versioned stdout errors with command identity, typed cursor categories and recovery hints. Human diagnostics and standalone count continuation retain their existing behavior.

Validation errors honor UTF-8 byte limits, including the final newline. If the complete error cannot fit, E028_RESPONSE_BUDGET_TOO_SMALL reports the measured minimum and preserves the original validation_error. Shared-wrapper regression assertions, including symbols NDJSON cursor validation, require structured machine output for malformed, mismatched and stale cursors with empty stderr.

Fixes #5412

Validation

• Repository build succeeded without warnings or errors.
• Release cursor, bounded-response, envelope, find, response-budget and related MCP regression tests: net8.0 510 passed; net9.0 501 passed, 9 skipped; no failures.
• The issue's CLI reproduction now returns exit 1 and structured stdout.
• A batch smoke check preserves the structured cursor error and emits no stderr.
• Modified C# whitespace verification and git diff --check passed.
• dotnet run --project tools/CodeIndex.Changelog -- check passed.
• Post-commit rebuild and root/workspace index checks passed.
• Codex adversarial review of origin/main..HEAD, including a follow-up review of the CI test correction: No blocking/actionable issues found.

Documentation and changelog

• Updated both language sections in docs/find-scan-controls.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5412.fixed.md; CHANGELOG.md is unchanged.
• Agent instruction files are unchanged.

Follow-ups

Tracked separately: #5423.