Create `airflow.core.*` module structure for Server-client separation

[kaxil]

Move the current server components under airflow.core.* module structure & namespace to establish clear boundaries for AIP-72 server-client separation to clearly mark server components.

Example:

• airflow.models.* → airflow.core.models.*
• airflow.jobs.* → airflow.core.jobs.*
• airflow.executors.* → airflow.core.executors.*
• airflow.api_fastapi.* → airflow.core.api.*

Success Criteria

• All server components in airflow.core.* namespace
• No internal code imports from old locations
• All tests pass with new structure
• Clear separation between server and shared components

[rich7420]

Hello , Could I try this?

[potiuk]

Same here - this is more of a "meta-task" rather than task that can be easily accomplished.

[rich7420]

thanks!

[amoghrajesh]

Self assigning this one, will pick it up soon.