fix: implement per-validator rate limit in slashing mempool

[dev-king-001]

Title:

fix: implement per-validator rate limit in slashing mempool

Description

What was done

• Added a HashMap<ValidatorIndex, u8> rate-limiter to the SlashingMempool.
• Enforced a hard limit of MAX_EVIDENCE_PER_VALIDATOR_PER_EPOCH = 1.
• Configured push_evidence() to immediately return an OverflowError and drop any redundant evidence for the same validator.
• Implemented reset_epoch() to clear the rate-limit map at epoch boundaries.
• Integrated the slashing module into the Soroban environment securely via #![cfg_attr(target_family = "wasm", no_std)].

Why it was done

• To prevent a known griefing vector where an attacker floods the mempool with minimal-evidence packages for a single victim validator. Previously, unbounded evidence processing could exhaust the epoch processing gas budget and permanently block legitimate slashing evidence from being evaluated.

How it was verified

• Validated via cargo test --test griefing_resistance_test.
• Added a DoS resistance test (test_minimal_evidence_flood_griefing_resistance) that actively pushes 100 duplicate evidence entries for the same validator index, verifying that exactly 1 is successfully accepted while the remaining 99 are immediately rejected with an OverflowError::RateLimitReached.

Closes #25

[JamesEjembi]

CI failed

[dev-king-001]

Fixed

[JamesEjembi]

LGTM