Test coverage gaps: error cases, Blazor bUnit

[StepKie]

Context

From the pre-1.3.0 code-review findings (.claude/findings.md was deleted in 2a7c89c; surviving items consolidated here).

What's missing

Parser/handler tests

• MtgCsvHelper.Tests/MtgCardCsvHandlerTests.cs:38 — Deckbox round-trip test commented out, tied to Moxfield to Deckbox conversion issues with some set names #31 (Deckbox set-name aliases). Re-enable when Moxfield to Deckbox conversion issues with some set names #31 lands.
• Error-path tests: invalid format names, missing required columns, completely malformed CSV. (Some now covered by FaultToleranceTests after Fault tolerance: partial conversion with per-row error reporting #40, but error-on-unknown-format and missing-column-header are gaps.)

Blazor component tests

• No bUnit tests for the MtgCsvProcessor component — the largest piece of UI logic in the project goes untested.
• Specifically valuable cases: input/output dropdown filtering (just added in this PR), auto-detect on file change, format swap when input == output, error UI for malformed CSVs.

Out of scope

• E2E tests through Playwright/etc. — the bUnit layer is the right next step before reaching for browser drivers.