Fix OpenAPI 3.1 spec validation errors

[andig]

Fixes 14 spec validation errors flagged by vacuum lint (quality score 10→69; remaining items are warnings/infos only).

Changes in server/openapi.yaml:

• Quote 200 response keys (status codes must be string keys)
• Replace nullable: true + type: X with type: ["X", "null"] (×4) — nullable is removed in OpenAPI 3.1
• Use examples array instead of singular example on LogLevel and Phases — 3.1 schemas follow JSON Schema 2020-12; the singular example caused enum mis-validation

server/mcp/openapi.json regenerated via go generate ./server/.

🤖 Generated with Claude Code

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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