feat: make stake_on_call participant-aware

[Meet-hybrid]

Overview

Make stake_on_call position-aware for multi-outcome calls by validating the selected pool and counting each staker only once per call.

Related Issue

Closes #317

Changes

🎯 Position-Aware Call Staking

• [MODIFY] Deduplicate participant_count across all UserStake outcome entries for the same staker and call.
• [MODIFY] Use checked participant-count arithmetic to prevent silent overflow.
• [ADD] Cover a repeat staker selecting a second outcome in the multi-outcome test while preserving all pool and stake totals.
• [PRESERVE] Existing pause, settlement, end-time, outcome-index, SAC transfer, surge-fee, vault, event, and persistent-TTL behavior.

Verification Results

cargo fmt --manifest-path packages/contracts-stellar/call_registry/Cargo.toml -- --check
✅ passed

git diff --check
✅ passed

cargo test --manifest-path packages/contracts-stellar/call_registry/Cargo.toml
⚠️ blocked: the environment could not resolve index.crates.io while fetching rand

Acceptance Criteria	Status
Multi-staker, three-outcome pools remain correct	✅ Existing multi-outcome coverage retained and extended
participant_count is not double-counted	✅ Checks all per-outcome UserStake entries before incrementing
Paused contract blocks staking	✅ Existing pause guard preserved

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[PeterOche]

@Meet-hybrid your pr is failing checks, please fix