Add unit tests for Instructions, Fees, and Clock sysvars

[anand9125]

Summary

Adds unit tests for three sysvars that previously had no tests:

• Instructions (sdk/src/sysvars/instructions.rs) – parsing and introspecting the Instructions sysvar
• Fees (sdk/src/sysvars/fees.rs) – FeeCalculator, FeeRateGovernor, and Fees construction and logic
• Clock (sdk/src/sysvars/clock.rs) – from_bytes / from_bytes_unchecked and layout constants

Instructions sysvar

• Helpers to build mock sysvar buffers (instruction blobs and full layout).
• Tests for num_instructions(), load_current_index(), load_instruction_at(), get_instruction_relative() (in bounds, out of bounds, negative index).
• Tests for IntrospectedInstruction: num_account_metas(), get_program_id(), get_instruction_data(), get_instruction_account_at(), and account flags (is_signer, is_writable).
• 9 tests total.

Fees sysvar

• Tests for FeeCalculator::new, FeeRateGovernor::default(), create_fee_calculator(), burn() (50%, 0%, 100%), and Fees::new().
• 7 tests total.

Clock sysvar

• Helper to serialize a Clock to bytes (repr(C) layout).
• Tests for Clock::LEN, from_bytes() (valid, too short, empty), from_bytes_unchecked(), and default constants.
• 6 tests total.

Testing

cargo test -p pinocchio

[anand9125]

Hey @febo pls review this pr