Implement `create_school_if_needed` Instruction

[sanaderi]

Create a School account on-chain as a Program Derived Address (PDA) when the first donation request for a school is submitted. This account will store minimal information and be owned by the program, ensuring users cannot modify it directly.

Tasks

• Define a School PDA that stores:
  • school_id (string or bytes)
  • Optional minimal metadata
• Implement logic to:
  • Check if a School PDA already exists for a given school_id.
  • Create the School PDA if it does not exist.
  • Ensure the PDA’s owner is the program.
• Ensure PDA initialization includes correct space, lamports, and bump seed.
• Write unit tests covering:
  • Creation of a new School PDA.
  • Attempting to create a duplicate School PDA (should not create a new one).

Acceptance Criteria

• School PDA is created automatically if it doesn’t exist.
• The program owns all School PDAs.
• No duplicate School PDAs are created.
• Unit tests pass successfully.

@holygrimacing