feat: Add close validator vault

[GabrielePicco]

Description

• Add close validator vault

Status	Type	⚠️ Core Change	Issue
Ready	Feature	No	-

Greptile Summary

Adds functionality to close validator fees vaults in the delegation program, including instruction handlers, discriminators, and comprehensive test coverage.

• Added CloseValidatorFeesVault discriminator (0xe) in src/discriminator.rs for identifying close vault instructions
• Added process_close_validator_fees_vault in src/processor/close_validator_fees_vault.rs with proper admin authorization and PDA validation
• Added instruction builder close_validator_fees_vault in src/instruction_builder/close_validator_fees_vault.rs for constructing close vault instructions
• Added integration test test_close_validator_fees_vault.rs verifying vault closure functionality

[greptile-apps]

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