Remove System.exit from main and error paths, add tests

[eskopp]

Changes

• MyCryptMain: logic moved to execute(args), main only calls it. A missing or unknown command now exits with code 1 instead of 0.
• RunEncrypt.getCipher: throws IllegalStateException instead of calling System.exit(8).
• RunGenKeys: a key file that cannot be written is reported as IOException instead of System.exit(2).
• New tests for MyCryptMain (usage, full cycle, copy) and for error paths (missing files, wrong key, unwritable path).

Coverage

JaCoCo: 62% -> 88% instructions, 71% branches, 86% lines. 19 tests, all green locally on JDK 27.

Not changed

The System.exit(2) for a missing RSA algorithm in RunGenKeys stays, it is unreachable in practice.