Fixed #1292

Introduce a new HTTP POST endpoint (/configuration-as-code/configure) to allow applying JCasC YAML directly from the request body.

Key features:

• Enforces Jenkins.ADMINISTER permission
• Requires POST method
• CSRF protection respected
• Supports API token authentication for automation use cases
• Validates request body and rejects empty payloads
• Returns structured JSON errors for invalid YAML or configuration issues

Test coverage includes:

• Permission enforcement (admin, non-admin, anonymous)
• CSRF handling (with/without crumb, API token bypass)
• Invalid YAML and malformed structure handling
• Empty request body validation
• Idempotent configuration application

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