test: cover mission validator contracts

[ahmdkaml]

Relates to #571

Adds focused unit tests for the documented MissionValidator contracts:

• Rejects empty mission IDs.
• Rejects missions without objectives.
• Rejects unsupported objective types.
• Rejects non-positive objective and reward counts.
• Rejects unknown objective targets and reward items.
• Verifies Mine objectives resolve known block targets.
• Verifies a valid mission produces no validation problems.

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[marceld23]

Checked every assertion against MissionValidator.cs and ran locally: 9/9 passing in ~1 s, Release, no warnings; CI green. There was no source-level test coverage for the validator before — good pick, since both the in-game player editor and the admin extension editor share these rules.

All seven problem-message assertions match the implementation verbatim, the Mine-resolves-blocks vs. everything-else-resolves-items branch is exercised from both sides, and the happy path pins Validate returning empty AND IsValid. The per-test Load() helper mirrors what ContentTests and the worldgen tests already do, so it fits the house style.

One recurring micro-nit (third PR in a row, so worth mentioning once): a blank line is missing between a closing method brace and the next [Fact] (here after Validate_AcceptsKnownGoodMission). dotnet format doesn't enforce it, so nothing fails — just a readability habit worth adopting. Merging.