Fail generate when --generator has no name, and accept --generator=Name [patch]

[matt-edmondson]

Fixes #259

What changed

GetOption only recognised --generator when its value came as a separate token. In two cases it returned null, which Generate reads as "no filter":

• the option was the last argument
• it was written --generator=Name

Either way, every generator ran and the command exited 0.

Generate now reads the filter through TryGetGeneratorFilter before it loads the schema or writes anything:

• --generator with no value, followed by another option (--generator --verbose), or written as an empty --generator= → '--generator' needs a code generator name., exit Failure, nothing written.
• --generator=Name → selects that generator, as the triage comment recommended.
• An absent option still means every generator, and --generator Name behaves as before.

FindSchemaPath already steps over --generator=Name as a single token, so it needed no change. Rejecting unknown options was optional in the issue and is left out.

Tests

• TestGeneratorOptionWithoutANameFailsAndWritesNothing (three data rows: --generator, --generator=, --generator --verbose) checks for Failure, the error text, and that no out/ directory is created.
• TestGeneratorOptionAcceptsTheEqualsForm: --generator=CSharp writes Item.g.cs, and --generator=Cpp fails with "no code generator named 'Cpp'".
• With the fix reverted, all 4 new cases fail. With it, they pass. The existing --generator tests still pass.
• dotnet test Schema.Test: 514/514 passing locally.

🤖 Generated with Claude Code

https://claude.ai/code/session_019FkGdWB8Ko3mTn9JiBH973

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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