feat: implement dynamic protocol fee discounts based on creator execu…

[Chrisbankz0]

#closes
#826

Summary

Implements volume-based protocol execution fee discounts for high-volume task creators.

Related Issue

Type of Change

• Feature
• Bug fix
• Refactor
• Documentation

Changes Made

• Added persistent per-creator successful execution counters.
• Added a 10% discount after 100 executions.
• Added a 25% discount after 1,000 executions.
• Added the FeeDiscountTierUpdated event for tier progression.
• Added get_creator_execution_count for counter inspection.
• Ensured failed executions do not increment creator counters.
• Added tests for discount tiers and creator counter isolation.
• Appended the new storage key to preserve existing Soroban storage indexes.

Validation

• Editor diagnostics report no errors.
• git diff --check
• cargo test
• cargo fmt --all
• Manual verification completed

Rust tests could not run locally because the Windows environment does not have the MSVC linker (link.exe) installed. The tests should be run after installing Visual Studio Build Tools with the C++ workload.

Recommended commands:

cd contract
cargo fmt --all
cargo test test_creator_execution_fee_discount_tiers
cargo test test_creator_execution_counts_are_isolated
cargo test

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